Updates for fixes on the core ec2 inventory module

Inventory syncs will still fail if you specify an instance_filter.  We
need to check:

https://github.com/ansible/ansible/pull/13178

For the moment I'm making sure we specifically disable elasticache
querying but it still yields an error.

Will follow up with:

https://trello.com/c/h1U0w4WC/138-ec2-module-is-busted-if-you-set-instance-filters

awx/plugins/inventory/ec2.py

@@ -505,7 +505,7 @@ class Ec2Inventory(object):
         # that's why we need to call describe directly (it would be called by
         # the shorthand method anyway...)
         try:
-            conn = elasticache.connect_to_region(region)
+            conn = self.connect_to_aws(elasticache, region)
             if conn:
                 # show_cache_node_info = True
                 # because we also want nodes' information
@@ -541,7 +541,7 @@ class Ec2Inventory(object):
         # that's why we need to call describe directly (it would be called by
         # the shorthand method anyway...)
         try:
-            conn = elasticache.connect_to_region(region)
+            conn = self.connect_to_aws(elasticache, region)
             if conn:
                 response = conn.describe_replication_groups()