Enable the ability to pass in a custom codename

While testing out Aptly, the `apt-get` client complains with the following error, since the `codename` was switched from the InRelease files that are baked out by Aptly:

```
E: Repository 'http://debianrepo.example.com/bionic testing InRelease' changed its 'Codename' value from '' to 'testing'
```

system/t06_publish/PublishSnapshot1Test_release_amd64

@@ -2,5 +2,6 @@ Origin: LP-PPA-gladky-anton-gnuplot
 Label: . maverick
 Archive: maverick
 Suite: maverick
+Codename: maverick
 Architecture: amd64
 Component: main

system/t06_publish/PublishSnapshot1Test_release_i386

@@ -2,5 +2,6 @@ Origin: LP-PPA-gladky-anton-gnuplot
 Label: . maverick
 Archive: maverick
 Suite: maverick
+Codename: maverick
 Architecture: i386
 Component: main

system/t06_publish/PublishSnapshot35Test_release_udeb_i386

@@ -2,5 +2,6 @@ Origin: Debian
 Label: . stretch
 Archive: stretch
 Suite: stretch
+Codename: stretch
 Architecture: i386
 Component: main

system/t12_api/publish.py

@@ -41,6 +41,7 @@ class PublishAPITestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'Origin': '',
@@ -86,6 +87,7 @@ class PublishAPITestRepo(APITest):
         repo2_expected = {
             'AcquireByHash': False,
             'Architectures': ['amd64', 'i386'],
+            'Codename': '',
             'Distribution': distribution,
             'Label': '',
             'Origin': '',
@@ -164,6 +166,7 @@ class PublishSnapshotAPITest(APITest):
         repo_expected = {
             'AcquireByHash': True,
             'Architectures': ['i386'],
+            'Codename': '',
             'Distribution': 'squeeze',
             'Label': 'fun',
             'Origin': 'earth',
@@ -249,6 +252,7 @@ class PublishUpdateAPITestRepo(APITest):
         repo_expected = {
             'AcquireByHash': True,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'Origin': '',
@@ -343,6 +347,7 @@ class PublishUpdateSkipCleanupAPITestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'Origin': '',
@@ -406,6 +411,7 @@ class PublishSwitchAPITestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'NotAutomatic': '',
@@ -449,6 +455,7 @@ class PublishSwitchAPITestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'Origin': '',
@@ -510,6 +517,7 @@ class PublishSwitchAPISkipCleanupTestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'NotAutomatic': '',
@@ -546,6 +554,7 @@ class PublishSwitchAPISkipCleanupTestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'otherdist',
             'Label': '',
             'NotAutomatic': '',
@@ -584,6 +593,7 @@ class PublishSwitchAPISkipCleanupTestRepo(APITest):
         repo_expected = {
             'AcquireByHash': False,
             'Architectures': ['i386', 'source'],
+            'Codename': '',
             'Distribution': 'wheezy',
             'Label': '',
             'Origin': '',