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: I45adc06dae0805bd401f7c16b703b2717e1ac46d
Merge "Update spec_helper to include 'chefspec/berkshelf'"