Update tenant inventory collection

Use the OpenStack Identity Service (Keystone) to collect the full list of
tenants in OpenStack installations.
Add migration and model for quotas

* Create cloud_resource_quotas table with STI
* Create CloudResourceQuota and OpenstackResourceQuota model
    - explicitly set tablename for CloudResourceQuota since rails believes that
      the plural of "quota" is "quota"
* Update CloudTenant and EmsCloud with quota relationships

Add migration and model for quotas

Add ems relationship to quotas

Explicitly set table name for cloud quotas

According to Rails, the plural of quota is quota.  But, to stay consistent, the
table name is overridden to be "cloud_resource_quotas".

Add quotas relationship to cloud tenants
Add quota inventory collection
Add attr_accessible to cloud_resource_quota

Allows update_attributes to set type and cloud_tenant_id