AS-265: delete server (review)

`DELETE .../groupId/servers/serverId` endpoint implemented that will delete specific server from the group. It takes optional `replace` query argument that tells whether to replace the deleted server by creating new one. The default is `True`.

Tested with [crest](https://github.com/manishtomar/crest) (Yes! I am going to brag it in every PR :D) and it works fine :)
> Hmm... do we want to test that it is the only thing in pending?

Oh yes. Very much :) Thanks for pointing out