Add quota support to Openstack Tenants

Adds support for collecting and storing openstack quota details form various openstack services.

Backend updates:
* Adds `cloud_resource_quotas` table and models
    * with STI to support OpenStack specific quotas
* Updates Openstack inventory collection
    * fixes the way that openstack tenants were collected
    * collects quotas based on services and tenants
        - e.g., in Tenant "ABC", the "compute" service allows for 30 vCPUs

UI Updates:
* Tenant tab in Cloud subnav
* Cloud Tenants in Openstack provider relationships lists (listnav and textual summary)
* Security Groups, Instances, and Images relationships for Tenant summary page
* Quota on Tenant summary page

NB:  Rebased this PR to remove the references to the Tenant Auth support.  I've got that stashed away as a separate branch for now.  If it turns out we still need that concept, we can go back to it.  For more information, see @roliveri's comments on outdated diffs below.

@Fryguy, @roliveri, @dclarizio, @h-kataria please review
As long as the EMS user is an admin (in any tenant) they can access the quota information for any tenant.  That's where the openstack ACLs get really crazy.

And, I think we're going to have to insist on an admin user, even if it's not "the" admin user.  The user needs to be an admin in order to access quota information regardless.  I tried without an admin, and it blew up with an unauthorized error.

I'll make the other change noted above (use `accessible_tenants` to get list of tenants) and that will at least limit the scope of what quota information we gather to the list of tenants our admin user has access to.