Check DB scheme prior to migration to Ml2

When using migration tool from LB/OVS plugin to Ml2, there is no
guarantee current scheme is supported by migration tool. This patch
checks version stored in DB by alembic and compares whether version
is supported.

Closes-bug: #1307720

Change-Id: I4519a0e5a0f3027675958a68d1f9e0440b177229
Merge "Check DB scheme prior to migration to Ml2"