Add TLS configuration

[tjcelaya]

As a result of the current implementation of #48, RPC traffic is exposed publicly and should therefore be encrypted. Since consul provides mechanisms to do so we should include a way to inject certs into the containers before consul can start in a similar fashion to how autopilotpattern/vault uses docker exec to bootstrap.

The proposed design is to check for CONSUL_TLS_PATH during preStart and if present, wait for a file to appear the the specified path. Gossip key configuration can be done by specifying CONSUL_ENCRYPT_PATH or CONSUL_ENCRYPT_BASE64.

[misterbisson]

This is implemented in https://github.com/autopilotpattern/vault/

[tjcelaya]

Right, the plan was to adapt the strategy describe in this section of the autopilotpattern/vault README to get the key material into the containers and use CONSUL_TLS_PATH to signal to ContainerPilot where the key will appear. Leaving that environment variable empty would indicate no TLS was being configured.