Support scope in dynamic cred for specific roles

We already added scope support for 'admin', 'member',
and 'reader' role. This commit is to adds the scope
support for specific roles, basically in get_creds_by_roles().

Test can now request the scope along with the number of roles
using the scope as prefix in credential type. Fpr example:

    credentials = [['my_role', 'role1'], # this will be old style and project scoped
                   ['project_my_role', 'role1'], # this will be project scoped
                   ['domain_my_role', 'role1'], # this will be domain scoped
                   ['system_my_role', 'role1']] # this will be system scoped

and below is how test can access the credential manager of respective
credentials type:

    cls.os_my_role.any_client
    cls.os_project_my_role.any_client
    cls.os_domain_my_role.any_client
    cls.os_system_my_role.any_client

Closes-Bug: #1917168
Change-Id: I9053faa255e3680d7f870e3cdedf62fb2eb5cb1a
8 files changed