add support for --subnet-id

ec2stack/providers/cloudstack/instances.py

diff --git a/ec2stack/providers/cloudstack/instances.py b/ec2stack/providers/cloudstack/instances.py
index 01d2127..7ed22ab 100644
--- a/ec2stack/providers/cloudstack/instances.py
+++ b/ec2stack/providers/cloudstack/instances.py
@@ -226,6 +226,9 @@
 
         args['securitygroupnames'] = ",".join(securitygroupnames)
 
+    if helpers.get('SubnetId') is not None:
+        args['networkids'] = helpers.get('SubnetId')
+
     args['command'] = 'deployVirtualMachine'
 
     response = requester.make_request_async(args)