Add an ability to cache auth token

Add option --os-cache, which defaults to OS_CACHE variable
(by default - False). When this option is specified, manilaclient
will use auth token saved in keyring. First time when command will
be called, auth token and user password will be saved in keyring.
Using --os-reset-cache option password and auth_token stored in the
keyring can be deleted.

Implements bp cache-auth-token

Change-Id: Ia8959655528acce726baa616e66901c7a96fd35c