Provide a new ComputeManager for Ironic

Ironic is internally clustered and that causes some friction
with Nova which assumes that each n-cpu instance is the only
one responsible for any given VM. While this is going to be
addressed in the long term, at the moment we need to make
it possible to run in an HA setup for Ironic.

The short term solution proposed is to run 2+ nova-compute's
each of which reports the same hostname (e.g. ironic). This
works but has some caveats - one of which is that _init_instance
will now race between different nova-compute instances starting
up at the same time. A custom ComputeManager permits us to address
that without prejuidice to future long term solutions.

Relatedly, TripleO needs Ironic to permit service startup before
keystone is initialised, which the removal of API calls during
init_host permits - and as there are no API calls needed for
correct behaviour of the Ironic driver, this is straight forward :).

See https://etherpad.openstack.org/p/ironic-nova-friction for
more discussion.

Change-Id: I68d46c4da8715df03c3a88393b55665dc57045a3
Closes-Bug: #1295503