Quick fix for broken tenant commands in admin CLI

[msterin]

During the last merge (threading support in vmdk_ops) we accidentally broke tenant commands in admin CLI. No, they did not have unit test coverage - and this will be addressed in a separate PR (Issue #668 - The same PR will also take care of re-using the get_auth_mgt() function from auth.py)

This PR applies a quick bandaid by defining the missing function and moving it (and related _auth_mgr) global to admin CLI code.

// @shaominchen @lipingxue

Tested: manually. Was throwing exception before the fix. Now does the following:

[root@localhost:~] /usr/lib/vmware/vmdkops/bin/vmdkops_admin.py tenant create --name oneTenant
tenant create succeeded
[root@localhost:~] /usr/lib/vmware/vmdkops/bin/vmdkops_admin.py tenant ls
Uuid                                  Name       Description  Default_datastore  VM_list  
------------------------------------  ---------  -----------  -----------------  -------  
2955e3ed-fec5-4d49-b878-ab0eb493704b  oneTenant               default_ds                  
[root@localhost:~] /usr/lib/vmware/vmdkops/bin/vmdkops_admin.py tenant rm oneTenant
tenant rm succeeded
[root@localhost:~] /usr/lib/vmware/vmdkops/bin/vmdkops_admin.py tenant ls
Uuid  Name  Description  Default_datastore  VM_list  
----  ----  -----------  -----------------  -------  

[root@localhost:~]

[pdhamdhere]

Fine as workaround. Looks like we will have to patch or do 0.81. CC/ @kerneltime

[andrewjstone]

LGTM