Update to Python3

Update to Python3

Functest containers leverage on Python3 instead of python2.
https://mail.python.org/pipermail/python-dev/2018-March/152348.html

It also updates robotframework librairies to latest release and Vmtp
to master ([1] is needed)

It patches cloudify rest client to support python3.

Vmtp is currently disabled because it currently supports python2 only.

[1] https://github.com/openstack/vmtp/commit/a5d062881d91bf4f547d92c6e289bea30feb5d6e#diff-b4ef698db8ca845e5845c4618278f29a

Change-Id: I39964a212ec2d4dbf53c9ea7c63f02e0b6a05b48
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>