Update spec_helper to include 'chefspec/berkshelf'

This minor change allows you to run rspec outside of strainer while
still having cookbook dependencies automatically handled by berkshelf
(which also means you can run individual chefspec tests).

Note that this change does marginally increase the time it takes to run
a strainer test.

If this change is accepted, then the remaining stackforge OpenStack
cookbooks will be updated also.

Change-Id: Ic50dcab83abd1937f9797444f36d477140abb183
Merge "Update spec_helper to include 'chefspec/berkshelf'"