Make user_enabled and tenant_enabled booleans

Currently, the register resource accepts a "true" or "false" string to
enable/disable both users and tenants. As the keystone schema has
changed, make these attributes booleans instead.
Update providers to make use of http library

The http logic was duplicated in both the credentials and register
providers. Move this to openstack-common in order to have a single
place for this to reside as well provide http functionality to other
cookbooks.