nova.virt.ironic passes ephemeral_gb to ironic

Make the nova.virt.ironic driver pass the ephemeral_gb flavor spec, if
present, to Ironic via driver_info/pxe_ephemeral_gb. Will also forward
the CONF.default_ephemeral_format value to Ironic.

This patch also adds a default ephemeral format CONF option to
ironic.drivers.modules.pxe, just in case this isn't sent from Nova, eg.
because the config option has no default value.

Closes-bug: #1307726
Change-Id: I445965540c95db7a80c7f8da88e9d4218dcc8f01
Merge "nova.virt.ironic passes ephemeral_gb to ironic"