modules/aws/vpc/sg-etcd: Split out aws_security_group_rule for etcd
modules/aws/vpc/sg-etcd: Split out aws_security_group_rule for etcd

This happend for masters and workers in b620c16f (modules/aws: tighten
security groups, 2017-04-19, coreos/tectonic-installer#264, where:

* Master ingress/egress rules moved from inline entries in
  modules/aws/master-asg/master.tf to stand-alone rules in
  modules/aws/vpc/sg-master.tf.

* Worker ingress/egress rules moved from inline entries in
  modules/aws/worker-asg/security-groups.tf to stand-alone rules in
  modules/aws/vpc/sg-worker.tf.

For some reason, b620c16f moved the etcd security group from
modules/aws/etcd/network.tf to modules/aws/vpc/sg-etcd.tf without
splitting out the inline rules, so this commit catches up for
consistency with the other node classes.  From the Terraform docs [1]:

  Terraform currently provides both a standalone Security Group Rule
  resource (a single ingress or egress rule), and a Security Group
  resource with ingress and egress rules defined in-line.  At this
  time you cannot use a Security Group with in-line rules in
  conjunction with any Security Group Rule resources.  Doing so will
  cause a conflict of rule settings and will overwrite rules.

We can also use the rule name to hint at the purpose of a rule, while
with inline rules we just have port numbers.  In this case, the *_etcd
and *_peer suffixes are based on [2]:

  The official etcd ports are 2379 for client requests, and 2380 for
  peer communication.

[1]: https://www.terraform.io/docs/providers/aws/r/security_group_rule.html
[2]: https://github.com/coreos/etcd/tree/v3.3.9#etcd-tcp-ports

derekhiggins/installer,derekhiggins/installer