compute: Move volume attached live migration tests to use SSHABLE
compute: Move volume attached live migration tests to use SSHABLE

Live migration tests the volume attachment and checking the
server ssh-able will help to avoid any race during attachment
operation.

Note we merged this once Icdb3398061d0274fec8b4e10d955a4b00edf4524 but
that was reverted in I005755b0e527255362923a6b4bbbba2f7c1dfbd1 as it was
broken. This patch tries to make it right the second time.

Change-Id: I898db6351e39fb4569e9fefeddf870fb8af0671c

openstack/tempest,openstack/tempest