Publish should be a no-op if no units and no settings have changed since the last publish

[ipanova]

Test that Publish should be a no-op if all of the following are true:

• no units have been added since the last successful publish.
• no units have been removed since the last successful publish.
• no distributor has been modified since the last successful publish.

For more info check https://pulp.plan.io/issues/1724
There is already implemented one test that is related #127 but i think that testing these 3 corner cases will be also good.

[ipanova]

One more corner case would be good to test when some values where specified in the override_config of the current publish - at this point publish should be operational.

[ipanova]

scenario1

1. create repo with feed, sync, publish
   2 assert publish is operational
   3 remove unit/add unit/ update distributor's config( like disable publish over http)
   4 publish
   5 assert publish is operational

scenario2
1 create sync repo, publish
2 assert publish is operational
3 publish repo and set override config
4 assert publish is operational

scenario3
1 create sync repo, publish
2 assert publish is operational
3 publish repo and set override config with values X
4 assert publish is operational
5 publish repo and set override config with values X( override config should be same as in step3)
6 assert publish is no-op

[midnightercz]

Tests in #290
scenario3 covered with NOOPTestCase1
scenario1 coverred with NOOPTestCase2
scenario2 seems to be subset of scenario3