Standardize usage of client arguments (install-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: Ib1806e86f11b3f2d5fba0035bb4f4094ee8f3d1c
Merge "Standardize usage of client arguments (install-guide)"