LU-7003 utils: must quote the value of the context option

LU-7003 utils: must quote the value of the context option

As per mount(8) notes, the context value might contain commas,
in which case the  value has to be properly quoted, otherwise
mount(8) will interpret the comma as a separator between mount
options.

Signed-off-by: Frederic Saunier <frederic.saunier@atos.net>
Change-Id: I75e958da26273fe2a15ccae01a1176c3549821b8
Reviewed-on: http://review.whamcloud.com/18294
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>