Add Pod, Service, Replication Controller terms
Add Pod, Service, Replication Controller terms

Add Pod, Service, Replication Controller terms and definitions to the
user guide as requested in the review of the previous terminology
submission.

Change-Id: I30012f85484044dbb5eb3c280f2fd96cd2adcddf

openstack/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum,jay-lau/magnum