Make OS_CLOUD be able to be configured

OS_CLOUD is used to communiate to CLI tools what cloud
credentials to utilize.
The change I86ffa9cd52454f1c1c72d29b3a0e0caa3e44b829
unfortunately set an explicit OS_CLOUD account which breaks
any jobs which are expecting a previosuly set OS_CLOUD which
may be different to work. For example, OS_CLOUD being set
as devstack-system-admin to facilitate Secure RBAC testing.

Change-Id: Iee900e552584dda622f57eea3508df48dff2e071
1 file changed