vpc: allow outbound tcp/22

Need to be able to ssh *out* of our hosts to communicate with GitHub for
private repos. Add egress TCP port 22 access to the NATSecurityGroup.
vpc: access hosts by private dns name

Return private DNS names that will be resolvale within the VPC. This way
we can define a general SSH config that will work for all hosts.
cfn template: commands hostnames within VPC

It's a bummer that enabling VPC DNS hostnames isn't supported by
CloudFormation yet, so we have to do this manual command hack like
we're already doing for ElastiCache. Helpful bit for configuring your
ssh as well.
cfn templ: up max for app servers from 2 to 6

You may want to deploy more app instances by default to start with.
cfn template: removing ssh stanza hint, too clunky to be useful
Merge branch 'master' of github.com:edx/configuration into feature/sef/refarch-vpc-dns-ssh
README: reflect general jumpbox ssh config
undo mistaken change to stage target group
Merge branch 'master' of github.com:edx/configuration into feature/sef/refarch-vpc-dns-ssh
Merge pull request #29 from edx/feature/sef/refarch-vpc-dns-ssh

cloudformation template changes for outbound SSH, internal DNS names