Update the alembic migration section in the developer FAQ
Update the alembic migration section in the developer FAQ

After we've introduced a migration that relies on CONF option
in commit 1514b2a2f6, following the instruction steps will lead
to an ImportError. Change direct usage of the alembic command to
calling ironic-dbsync from the virtualenv.

Change-Id: I28e51e45c4e5cdb223795dbb4d720d233cd57808

openstack/ironic,pshchelo/ironic,NaohiroTamura/ironic,SauloAislan/ironic,openstack/ironic,pshchelo/ironic,NaohiroTamura/ironic,SauloAislan/ironic