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.

A similar change was recently accepted in the openstack-compute
cookbook.

Change-Id: Ia5b5f3cc8aab853dd0e8c3b120ed178b0f0887db