LBaaS V2 API and object model definition

Reproposing and repurposing the lbaas object model refactor into what
currently exists in the LBaaS v2 feature branch without the TLS and L7
rules.

APIImpact
DocImpact

Change-Id: I45b4875080ffa835817b2512948f3b039bf8b20c
Merge "LBaaS V2 API and object model definition"