Fixed alembic migrations to be correct tables.

Change-Id: Id67d29f8703d3e238d46e9f005a52a4dd395d070
Moved loadbalancerplugindbv2 to its own module and added models.  Minor adjustments to migration scripts.

Change-Id: I57fade6e9f73d05caa1d61905cf2d1db57a875ad
Changed prefix from lbv2 to lbaas. Changed name of v2 models that clashed with v1 models.  Minor fixes to alembic migrations.

Change-Id: I7be359a854e5ce00851070c4c10a7b453acd1bad