Handle setting id (uuid) in tempest_config

Tempest need uuids to be configured for some parameters like image_ref,
public_network_id. The types which handle name to id conversion
(tempest_glance_id_setter, tempest_neutron_net_id_setter) are not adding the
configuration in appropriate section but adding right to the bottom.

This patch:

* Add changes to tempest_config so it can handle setting uuids from given value.
  * Added a parameter named *set_id* with appropriate values to decide whether
    to grab id from the value given.
  * Changed the provider to get the id from given value on basis of set_id.
  * If set_id is not set, the value will be written.
  * Currently support grabbing glance_image_id and network_id
* Changed init.pp to
  * call tempest_config with appropriate set_id param where it is required.
  * Removed execution of tempest_glance_id_setter and
    tempest_neutron_net_id_setter which are not required.
when would image_id == :absent?