gluster: Corrected error messages

Corrected the error messages for stop and reset volume cases.
Earlier message used to be "Are you sure want to stop the following
Volume(s)?". The same is corrected to "Are you sure you want to stop
the following Volume(s)?"

Change-Id: Icc25f44b40b2a7e5ea5f84c17bb9179433803506
Bug-Url: https://bugzilla.redhat.com/960916
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>