Port os-server-groups extension to work in v2.1/v3 framework

Ports v2 os-server-groups extension and adapts it to the v2.1/v3 API
framework. API behaviour is identical with the exception
that there is no support for XML. Also

- unittest code modified to share testing with both v2/v2.1
- Adds expected error decorators for API methods
- Adds api sample tests

Partially implements blueprint v2-on-v3-api

Change-Id: I52e49e6e85a7516e3b184601374e5c987f60a9a0
Merge "Port os-server-groups extension to work in v2.1/v3 framework"