Port limits extension to work in v2.1/v3 framework

Ports v2 limits extension and adapts it to the v2.1/v3 API
framework. API behaviour is identical with the exception that
there is no support for rate limiting since no one uses that
seriously anyway.

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

Partially implements blueprint v2-on-v3-api

Change-Id: I072d364f4d2a92a02801862435f11417956d2dc0
Merge "Port limits extension to work in v2.1/v3 framework"