Add support for service plugins to migration framework

Some db migrations need to be run only if particular service is enabled (i.e. service plugin is configured in neutron config). Currently such migrations specify 'migration_for_plugins = ['*']' which is not quite correct.
Also following bugs are possible: https://bugs.launchpad.net/neutron/+bug/1209151 - agent extension migration is applied for particular core_plugins while lbaas agent scheduler migration for all plugins.
All this can be fixed by adding service plugins support to the migration framework.