Standardize usage of client arguments (user-guide)

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

Change-Id: If0a33ca02cd499769d1540b74b1ab0a217c839d4
Merge "Standardize usage of client arguments (user-guide)"