Remove redundant CLI arguments for neutron-db-manage
neutron.conf is found by the virtue of oslo.config auto-discovery
mechanism; and plugin.ini is no longer needed since Juno because now
schema does not depend on plugin used.
While at it, switched head -> heads to reflect recent changes in neutron
with multiple alembic branches. The old format still works, but 'heads'
is slightly more encouraged.
Change-Id: I614a6d43087fa231f0d582bab10a82480aaefda5
Related: Icc4de9824ef95781a1d060534973c2bbf8e03059