Fix Chef cookbook-related strainer issue

The last change* sadly missed a call to berks install and hence makes
strainer fail every time.  This change introduces a berks install call
to a workspace-relative directory that is then passed to strainer.

To use strainer, we need chef-berkshelf-prep for each test suite, though.

*) I0353b8207abb116ea3849c04df39546f8c2625db

Change-Id: Ic10fc436fae2e5bcc05f9fae15e43b7601255471
Merge "Fix Chef cookbook-related strainer issue"