Add pre/post-process options in beluga file

[leaexplores]

This is a tracking issue for a potential pre-process and post-process script option in the beluga file.

We could add something like
PRE_PROCESS_SERVER="myPreProcesScript.rb/.sh./.whatever"

POST_PROCESS_SERVER="myPreProcesScript.rb/.sh./.whatever"

PRE_PROCESS_TARGET="ip or all"

POST_PROCESS_TARGET="ip or all"

This would allow for easy integrations with tools that requires post deployment steps such as running rails migrations after deploying to a few host.