Document --plugin-paths/RALLY_PLUGIN_PATHS
Document --plugin-paths/RALLY_PLUGIN_PATHS

These options were added in 71d01a but were never added to the
docs. This removes the five duplicate places that plugin placement was
discussed, and replaces them with a single place that documents the
"new" options.

Change-Id: Ief27578692b220895027291d02f5e204351c8c0f

group-policy/rally,aforalee/RRally,amit0701/rally,eonpatapon/rally,openstack/rally,gluke77/rally,eayunstack/rally,paboldin/rally,gluke77/rally,vganapath/rally,group-policy/rally,openstack/rally,eonpatapon/rally,redhat-openstack/rally,afaheem88/rally,amit0701/rally,aforalee/RRally,afaheem88/rally,openstack/rally,vganapath/rally,eayunstack/rally,openstack/rally,vganapath/rally,cernops/rally,paboldin/rally,vganapath/rally,yeming233/rally,eayunstack/rally,gluke77/rally,cernops/rally,amit0701/rally,group-policy/rally,paboldin/rally,yeming233/rally,gluke77/rally,redhat-openstack/rally