Adds a USE_VAR_STORE parameter to enable the file based var-store

[garethjevans]

This is defaulted to true, setting it to false will disable the var-store and allow us to use credhub.

[cfdreddbot]

Hey garethjevans!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/154406718

The labels on this github issue will be updated when the story is started.

[garethjevans]

Fixes #49