Added rexec to allow execute script inside of container.

[zsuzhengdu]

Besides allowing operator to execute script on the path where maestro-ng runs, it would be nice to add the feature that allows operator to execute the script inside of running container.

Therefore, a check script could be added to check the status of 'required' service before starting. For example, check whether 'zookeeper' is fully stated before starting kafka.

  kafka:
    image: docker.cenx.localnet:5000/kafka:latest
    requires: [ zookeeper ]
    lifecycle:
      running: [{type: rexec, command: "python check.py"}]
    instances:
      kafka1:
        ship: machine
        ports: {client: 4082}
        lifecycle:
          running: [{type: tcp, port: client}]

NOTE: 'check.py' exists inside 'kafka' container.

[mpetazzoni]

Please also provide documentation for this new lifecycle helper in docs/lifecycle_helpers.rst.

[mpetazzoni]

@zsuzhengdu ping?

[mpetazzoni]

Nice, this is cleaner. But it doesn't look like it's waiting for the script to finish executing. If it's still running, it will just return false and another process will be started the following second.

[mpetazzoni]

Awesome. I'm assuming you're tested this? Do you mind squashing your changes into a single commit so I can merge a clean commit?

Thanks!

[zsuzhengdu]

I have tested this feature as far as I could.

One clean commit has been pushed as per request.

Thank you for your help for reviewing this PR!

[mpetazzoni]

Awesome; thanks!