Basically, there are three main installation scenario:

Scenario 1:
Ironic installation together with other openstack services
including keystone. In this case variable enable_keystone
is set to true and keystone service will be installed
together with ironic installation. It is possible realise this
scenario, no fix needed

Scenario 2:
Ironic installation with connection to already installed
keystone. In this scenario we have to set enable_keystone
to “No” to prevent from new keystone service installation
during the ironic installation process. But in other hand,
we need to have correct sections in ironic.conf to provide
all information needed to connect to existing keystone.
But all sections for keystone are added to ironic.conf only
if enable_keystone var is set to “Yes”. It isn’t possible
to realise this scenario. Proposed fix provide support for
this scenario, where multiple regions share the same
keystone service.

Scenario 3:
No keystone integration. Ironic don't connect to Keystone.
It is possible realise this scenario, no fix needed

Proposed solution also keep the default behaviour: if no
enable_keystone_integration is manually defined by default
it takes value of enable_keystone variable and all behaviour
is the same. But if we don't want to install keystone and
want to connect to existing one at the same time, it will be
possible to set enable_keystone var to “No”
(preventing keystone from installation) and at the same
time set ironic_enable_keystone_integration to Yes to allow
needed section appear in ironic.conf through templating.

Change-Id: I0c7e9a28876a1d4278fb2ed8555c2b08472864b9