supports alembic migration for db2
supports alembic migration for db2

This patch adds the optional runtime dependency to support db2
for alembic migration, will not break the default mysql path.

And I did the testing in db2 env, works fine.

Change-Id: I59deff0d82bc9e2e7ceaa157c626cd2bb7a36f0d
Closes-Bug: 1444270

dims/ironic,NaohiroTamura/ironic,SauloAislan/ironic,devananda/ironic,naototty/vagrant-lxc-ironic,naototty/vagrant-lxc-ironic,NaohiroTamura/ironic,bacaldwell/ironic,dims/ironic,openstack/ironic,redhat-openstack/ironic,hpproliant/ironic,supermari0/ironic,pshchelo/ironic,Tan0/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,naterh/ironic,SauloAislan/ironic,ionutbalutoiu/ironic,openstack/ironic,pshchelo/ironic