Use different keyring in system tests.

system/gpg-gen-key

@@ -0,0 +1,13 @@
+Key-Type: DSA
+Key-Length: 1024
+Subkey-Type: ELG-E
+Subkey-Length: 1024
+Name-Real: Aptly Tester
+Name-Comment: don't use it
+Name-Email: test@aptly.info
+Expire-Date: 0
+%pubring aptly.pub
+%secring aptly.sec
+# Do a commit here, so that we can later print "done" :-)
+%commit
+%echo done