Change Ironic bm flavor to use ephemeral partition

Adds use of the IRONIC_VM_EPHEMERAL_DISK variable to devstack
that will alter the flavor to specify an ephemeral partition
size. This change creates a 1 GB ephemeral partition, which
will be deducted from the overall disk size (IRONIC_VM_SPECS_DISK).

Change-Id: I37374d3fa8fd1b5cd194ac9659044031a3cad19e
Merge "Change Ironic bm flavor to use ephemeral partition"