Clarify deprecation of EXTRA_xxx_OPTS

The various EXTRA_xxx_OPTS variables will be removed in the Juno
development cycle, change the README to reflect the new way for
the Neutron variables.

Change-Id: Ic84da4a9b5a83e66cf0b57d643a87691f15517f0
Merge branch 'master' of git://github.com/openstack-dev/devstack