Fix the "Gnocchi" link pages of custom.rst doc
Fix the "Gnocchi" link pages of custom.rst doc

The "Gnocchi" link in the install doc named custom.rst deprecated
as http://gnocchi.readthedocs.io/en/latest/basic.html. So it
should be up-to-date for users.

Change-Id: I3d71dd44345a673829394a74921815698b636887
Closes-Bug: #1627910

openstack/ceilometer,ityaptin/ceilometer,ityaptin/ceilometer,openstack/ceilometer