Modify NO_GCE_CHECK to be a global variable that default on an env var

Following @craigcitro's suggestion

oauth2client/client.py

@@ -94,7 +94,7 @@
 DEFAULT_ENV_NAME = 'UNKNOWN'
 
 # If set to True _get_environment avoid GCE check (_detect_gce_environment)
-NO_GCE_CHECK = False
+NO_GCE_CHECK = os.environ.setdefault('NO_GCE_CHECK', 'False')
 
 class SETTINGS(object):
   """Settings namespace for globally defined values."""
@@ -974,7 +974,7 @@ def _get_environment(urlopen=None):
     SETTINGS.env_name = 'GAE_PRODUCTION'
   elif server_software.startswith('Development/'):
     SETTINGS.env_name = 'GAE_LOCAL'
-  elif NO_GCE_CHECK is not True and _detect_gce_environment(urlopen=urlopen):
+  elif NO_GCE_CHECK is not 'True' and _detect_gce_environment(urlopen=urlopen):
     SETTINGS.env_name = 'GCE_PRODUCTION'
 
   return SETTINGS.env_name