Add a unit test for the Gluster storage type

Add a unit test for the Gluster storage type

This patch, extends the 'ganeti.utils.storage_unittest.py' unit test
with a new test for the Gluster storage type. Also, the current unit
tests are updated accordingly to correspond to the latest changes.

Signed-off-by: Dimitris Bliablias <bl.dimitris@gmail.com>
Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

Cherry-picked from 42fdf9d0d79fb84796ad40ff6eb8c95e3206263e

Signed-off-by: Lisa Velden <velden@google.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>