Add ENV default for bug 585

Per Bug 585, this uses an ENV variable as the default value if one is
not used to instantiate N2VC.

If neither is present, N2VC will raise an exception on calls to
CreateNetworkService and DestroyNetworkService.

Change-Id: Ia5761c7eb1954ffb7a3a46390e850de059f0d2a4
Signed-off-by: Adam Israel <adam.israel@canonical.com>