This commit adds running the plugin registry generation script to the
sphinx build. This ensure that every time we build the docs we'll have
an up-to-date plugin registry. This comes at the cost of increased
build time since the script will query openstack's git server to get
the plugin list. Also, the plugin-registry source file was added to
.gitignore since it is autogenerated now.
Change-Id: Ib26124c5d5d5ef4617e4e702e594ce7f1795ce23
fc46326854