avoid parsing heat.conf for each Heat resource

This moves heat configuration parsing to module loading time.

Parsing the config for each resource does not seem to be
useful, and profiling with pyflame shows that it is a significant
hit, measurable in tens of seconds on heat stack with a large
number of Contrail heat resources.

Change-Id: Iad81d60e63fdfb554e6c42dbcdf58848586984f1
Closes-Bug: 1779078

contrail_heat/resources/contrail.py

@@ -13,6 +13,8 @@
 
 LOG = logging.getLogger(__name__)
 
+cfg_parser = ConfigParser.ConfigParser()
+cfg_parser.read("/etc/heat/heat.conf")
 
 def set_auth_token(func):
     def wrapper(self, *args, **kwargs):
@@ -34,8 +36,6 @@ class ContrailResource(resource.Resource):
 
     def __init__(self, name, json_snippet, stack):
         super(ContrailResource, self).__init__(name, json_snippet, stack)
-        cfg_parser = ConfigParser.ConfigParser()
-        cfg_parser.read("/etc/heat/heat.conf")
         self._user = self._read_cfg(cfg_parser,
                                     'clients_contrail',
                                     'user',