Get properly env vars or their default values

Get properly env vars or their default values

It defines env.get() as an unique way to get Functest env vars or
their default values. It can be considered as a wrapper above os.environ.

It enforces backward compatibility via CONST which mustn't be used
for that purpose. It should be noted that it also stops using CONST
for getting OpenStack env vars.

Change-Id: I333dc1afbc0123166a7eaff8b551370098efa341
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>