etcd: implement separate system tests

- add t13_etcd test directory
- etcd will be started for the unit tests and each system test
- etcd will load fixture DB export if requested by the test
- existing tests are reused for etcd testing

system/t13_etcd/__init__.py

@@ -0,0 +1,3 @@
+"""
+Testing aptly with etcd DB
+"""