Add support for a forums application and a mongo database.
Give Edxapp instance s3 and ses permissions.
Minor Worker application related fixes.

Make sure the workers reference the correct wait handle and are in the RDS security group.
Updates to change the device names and update network acls.
Default to 3 mongo instances so that clusters do not need an arbiter.
Fix worker wait condition to wait for worker servers.
Remove scaling policies and scaling alarms.
Use a for look for repetative code.
Update instance tags to match new scheme.
Add the worker to the elasticache security group.
VPC DNS and Elasticache support

I hope this helps cleaning up a bit.
Corrections and fixes to match what the manual command used to do.
Fix cache references and correct subnet names.
Remove EdxServerUser since we have IAM roles setup for all application instance types.
Add a third subnet for mongo in a third AZ since there will be at minimum 3 instances.
Begin using us-east-1d AZ since we can't create subnets in us-east-1a
Merge pull request #346 from edx/feanil/forums_cfn

Feanil/forums cfn