Use file contents for stack name

[zaksoup]

Feature request: I'd like to be able to use the contents of a file for the stack name. I would like to be able to use a file containing a job name generated by a prior job step.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/116486999.

[mmb]

We will look into adding a new parameter called stack_name_file. If present the stack name read from this file will take precedence over the stack_name parameter.

@mmb and @cbguder