Adds a new flag, 'enable_openstack_core', which defaults to 'yes'.
Setting this flag to 'no' will disable the core OpenStack services,
including Glance, Heat, Horizon, Keystone, Neutron, and Nova.

Improves the default configuration of OpenStack Ironic when used in
standalone mode. In particular, configures a noauth mode when Keystone
is disabled, and allows the iPXE server to be used for provisioning as
well as inspection if Neutron is disabled.

Documentation for standalone ironic will be updated separately.

This patch was developed and tested using Bikolla [1].

[1] https://github.com/markgoddard/bikolla

Change-Id: Ic47f5ad81b8126a51e52a445097f7950dba233cd
Implements: blueprint standalone-ironic