[API][ADMIN_API] Support MySQL clusters

The API and Admin API server can now connect to multiple servers and
picks one at random on each access.  If one fails SQLAlchemy
automatically tries 3 times to get a new connection.

Change-Id: I961eee18fb402735684e2cfb83fb5dc416e46f87
[API][ADMIN_API] SQLAlchemy bind cleanups

Change-Id: I88b62f7dce663fe1b62b657cc5b51d7fdce60e33