Add support for ephemeral disk to ironic

Add a new variable, IRONIC_VM_EPHEMERAL_DISK, to specify the size
of the ephemeral disk. Default will be zero to match the current
behavior.

Change-Id: Ia8a8ef364a85fc803b93fb936212de0894a19bd8
Merge "Add support for ephemeral disk to ironic"
Merge branch 'master' of https://github.com/openstack-dev/devstack