webadmin: fix add-bricks validation in volume creation

If the user chooses 'Distributed Replicate' or 'Distributed Stripe'
during volume creation, an error will be showm if the brick count
is not equal to multiple of replica or stripe count respectively.

Change-Id: I3adc50b09f7c246bda0fcd63c906a8c726f5a3aa
Bug-Url: https://bugzilla.redhat.com/886478
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>