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

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

Change-Id: Idfc1124b1261a4b74fbdab524d92e54ef75662bb
Merge "Standardize usage of client arguments (admin-guide-cloud)"