Leverage on pbr (Python Build Reasonableness)

Leverage on pbr (Python Build Reasonableness)

It also updates requirements according to OpenStack stable/ocata
except for nova client version as snaps needs nova support of
floating IP which has been removed in pike [1] (8.0.0 [2]).

It must be reverted to >=6.0.0,!=7.0.0 as soon as snaps calls the
related neutron commands.

[1] https://github.com/openstack/python-novaclient/blob/master/releasenotes/notes/pike-rm-deprecated-net-272aeb62b329a5bc.yaml
[2] https://docs.openstack.org/releasenotes/python-novaclient/unreleased.html

Change-Id: Iaa0285471da5f2f73cbfef038c9902e1c39fa009
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>