restapi: Only one method to set mapping locator

The method that sets the mapping loator has been recently moved to the
backend base resource class, so it is no longer necessary to have it in
other resources and to explicitly check the type of the resource when
calling it.

Change-Id: I28595e36feb41bf2915e2fb0b28f78d7b3b65f74
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>