Refactor TenantManage

Refactor TenantManage

Refactor TenantManage to make the following changes:

1. Block on the retrieval of users and roles since
   the MD-SAL API is async.  Without this the calls
   will always return null since the async calls
   will not have completed yet.

2. Added new APIs to return users and roles as Maps
   to avoid linear searches.

Change-Id: Ib3ce98a3ac6adbbc66627e497d85531d12258c99
Signed-off-by: Gary Wu <gary.wu1@huawei.com>