Allow compatibility with Berkshelf 2.x and 3.x

openstack cookbooks currently use Berkshelf 2.x and we are
moving to 3.x.  Berkshelf deprecated the "berks install --path" cli
in favor of "berks vendor <path>".   While we transition all the
cookbooks, need to support both versions of Berkshelf.

Change-Id: I82a203fdb9b0c2ec47daefecb361d22aca306afb
Addresses: blueprint update-berkshelf
Merge "Allow compatibility with Berkshelf 2.x and 3.x"