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
(cherry picked from commit de10d7299ba3dce18b8e12a41830ec0e686df340)
Merge "Check DB scheme prior to migration to Ml2" into stable/icehouse