gluster: Log details of option being set

Modified the audit log message for setting a volume option
to include the key and value of the option, so that user
knows exactly what option was set to what value.

Change-Id: I7c5b4712128056a4ba5f1d87cf8fd9114f95116b
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>