make gluster volume name configurable

changed code to look at configuration file for
the metadata gluster volume name. The default is still
gsmetadata

Change-Id: I9bf74b9566ea1c9716c42f7ced0f999e02824868
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/6225
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/6258