Read the message file and execute all cexec steps

[rageshkrishna]

Addresses #2

This flow was tested end-to-end using mexec by publishing the following message:

{
  "steps": [
    {
      "name": "build.boot",
      "who": "mexec",
      "scripts": ["#!/bin/bash\n\ncexec_image_id=$(docker run -d --privileged=true --restart=no --net=host -e SHIPPABLE_API_TOKEN=cc540ee6-1d2c-43a4-8413-39facb15fab9 -e SHIPPABLE_API_URL=https://172.17.42.1:50000 -e SHIPPABLE_API_RETRY_INTERVAL=3 -e RUN_MODE=dev -e SHIPPABLE_AMQP_URL=amqp:https://SHIPPABLETESTUSER:[email protected]:5672/shippable -v /usr/bin/docker:/usr/bin/docker -v /var/run/docker.sock:/var/run/docker.sock -v /tmp/cexec:/cexec -v /home/shippable/cexec:/home/shippable/cexec:rw -t shipimg/cexec)\n\ndocker wait $cexec_image_id\n\necho __SH__SCRIPT_END_SUCCESS__"]
    },
    {
      "name": "build.ci",
      "who": "cexec",
      "scripts": ["ls -ltra; sleep 15; echo __SH__SCRIPT_END_SUCCESS__"]
    }
  ]
}

mexec received the message, successfully launched a cexec container and waited until it returned before acknowledging the message. The wait was accomplished using docker wait in the boot script right after launching the cexec container.

cexec correctly read the script file from the mounted /cexec directory and executed its script.