gluster: add force support to add-bricks

'create volume' and 'add-brick' operation in gluster will fail,
if the brick directories are in root partition.
'force' option added to skip the above check if required.

It can used to clear the xattrs on the bricks if the user
wants to re-use an existing brick.

Change-Id: I65b3e96d759a0b09b625da793b6c63b93f8c2131
Bug-Url: https://bugzilla.redhat.com/1008942
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>