Standardize usage of client arguments (user-guide-admin)

Like documented at https://wiki.openstack.org/wiki/Documentation/Conventions#Client_arguments:_.22--option_ARGUMENT.22
we prefer to use '--option ARGUMENT'.

Change-Id: I1a3a42cda0fd53577cec49a618d1ae84f504ece1
Merge "Standardize usage of client arguments (user-guide-admin)"