Properly quote the values of keystone arguments

To avoid shell expansion e.g. when a value contains a '$' sign.

Fixes: https://bugzilla.suse.com/show_bug.cgi?id=896750
(cherry picked from commit 17fd18dd571b44d9ab58b41048c2c3c7a0fe2250)
Merge pull request #156 from rhafer/stoney-backport

Properly quote the values of keystone arguments