Set default value for IPv6 subnet attribute flag

This patch would set the IPv6 subnet extended attributes flag
to True. The following thread has some additional information.
https://review.openstack.org/#/c/96723/

Change-Id: Ie6ac42646cc2a4d808c05126bf7dd51f2b03ee6b
Partially implements: bp tempest-ipv6-attributes-support
Don't set nova api/conductor workers since nova handles this

The changes to devstack to set nova api/conductor workers to 4 predated
commit 75c96a48fc7e5dfb59d8258142b01422f81b0253 to default the workers
equal to the number of CPUs, so this change just removes the hard-coding
from devstack and relies on nova's defaults for number of workers.

Change-Id: I384fc9eeb7a7eddf4679a6852203307a92d88a93
Setup color logging for Ironic

If enabled, setup colorized logging for Ironic API and Conductor.

Change-Id: I8c6d60185dd7907a0f9aa46fd1ae7a92f53406d0
Merge "Setup color logging for Ironic"
Merge "Don't set nova api/conductor workers since nova handles this"
Merge "Set default value for IPv6 subnet attribute flag"
Support multiple Cinder backend types

This is the first step in supporting multiple Cinder backend types at
once.  It initially converts the existing hard-coded multi-lvm support
to a new cinder_backends driver form.  Eventually the cinder_plugins
will be converted to this form so they can be enabled more than just
one at a time.

The default configuration should be identical to the previous defaults,
including for both True and False values of CINDER_MULTI_LVM_BACKEND.

The existing cinder_plugins are expected to be removed when this is
complete. They should continue to work until they have been converted.

Change-Id: I93b8ef32832269d730c76a6dc24ddb4f20c6d9df
Merge branch 'usb-prereq' into dt-test
Merge branch 'conf-tempest' into dt-test