Remove ephemeral size assertion in baremetal scenario

The test currently fails if the the baremetal flavor has no
ephemeral disk set.  This removes an unnecessary assertion and
unblocks tests against environments that do not deploy ephemeral
disks.

Change-Id: I7f13d4d7b40ea8879eaa30a19c117aef284586ff
Merge "Remove ephemeral size assertion in baremetal scenario"
Merge branch 'master' of https://github.com/openstack/tempest