Proposal - Include option to update policyfile instead of just install

[clickthisnick]

Similar to the always_update config in test-kitchen I think chefspec should have a similar option that allows this code to run a chef update instead of a chef install.

Chef update behind the scenes calls chef install.

The usecase of this is if I have a committed policyfile lock that is incorrect, then my unit tests fail, I need to either remove the policyfile lock or run chef update before I can run my tests