[config-ref] Convert ch_imageservice to RST
[config-ref] Convert ch_imageservice to RST

Change-Id: I0142b89bff3c932cedca8a2cb8e58b042dc5d195
Implements: blueprint config-ref-rst

AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals