Add split-stack environments

Add split-stack environments

Add 2 new environments to faciltate deploying split-stack:
environments/overcloud-baremetal.j2.yaml
environments/overcloud-services.j2.yaml

The environments are used to deploy 2 separate Heat stacks, one for just
the baremetal+network configuration and one for the service
configuration.

In order to keep Heat's view of the server's hostname consistent across
the 2 stacks the 2 environments set the same HostnameFormat with
"overcloud" as the stack name.

implements blueprint split-stack-default

Change-Id: I0b3f282c08af6fecea8f136908b806db70bada46