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 :)
> Would it make sense to add a flag to make sure that this can only ever be compared once?

I am not sure if this is required. Are you suggesting to raise an error if `__eq__` is called again? I can't think of situation when that will happen and even then I think caller should be interested in last match?  

> Also, should a similar format string be produced for __ne__?

Will `return not self == other` implementation for `__ne__` help? This way only `eq` needs to be implemented.