Cleanup manilaclient.utils module (part 2)

Remove functions 'arg', 'env' and 'add_arg' from
manilaclient.utils and use same functions from cliutils instead.

Partially implements: blueprint use-common-code

Change-Id: Ib41250d527c5949cdfb7c6956703ff4b371a062b
Merge "Cleanup manilaclient.utils module (part 2)"