Create agents table when ML2 core_plugin is used

In Icehouse agents table is not created when ml2 is used but later in
migration chain is added uniq constraint on this table. This patch makes
agents table created when ml2 plugin is used as core_plugin.

Closes-bug: 1260224
Change-Id: I501534261efd278b6a69a66758f1a01aa49bfa17
Merge "Create agents table when ML2 core_plugin is used"