webadmin: allow bricks in root partition

Added a new checkbox to 'allow' the brick directories
can be created a in the root partition.
This is applicable to adding bricks during volume creation
and later.

Selecting this option will send a 'force' parameter to
backend to bypass the 'root partition' performed by gluster.

Change-Id: I55ba2d05422f0a09c242e22e8ae3467e710065b7
Bug-Url: https://bugzilla.redhat.com/1008942
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>