Functional tests should use os_client_config for authentication

[Madhu Mohan Nelemane]

os_client_config provides simpler authentication mechanism. This is already in use in other openstack services (eg: python-novaclient). This shall also eliminate the need for a separate etc/manilaclient/manilaclient.conf which is used only by the functional tests.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/267018

[Goutham Pacha Ravi (gouthamr)]

Removed assignee because this bug has no activity for a long time.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Goutham Pacha Ravi (<email address hidden>) on branch: master
Review: https://review.opendev.org/267018
Reason: This is a pretty cool idea, but, its quite simple to allow configuration of this right now; if someone's interested in continuing this effort, we can revive this change.