Description of problem:
If one or more of the brick directories specified in a "gluster volume" create" command don't exist, the command will create those directories and then proceed to create the volume.  I believe that if any directories are missing, the command should fail.

Steps to Reproduce:
1. Create /bricks/b1 on each server.  Ensure this directory is empty on all servers.
2. Issue a command to create a volume using brick directories that do not exist.
gluster volume create testvol replica 2 transport tcp srv1:/bricks/b1/vol srv2:/bricks/b1/vol
3. Note that the vol directory now exists on all servers and the volume has been 

Expected results:
It should have refused to create the volume, with a helpful error message indicating which directories did not exist.