Remove unnecessary metadata check

Remove metadata cookbook check from chef-berkshelf-prep as it is
not necessary for this prep step and causes the gate-openstack-chef-repo
job to fail as the repo does not contain a metadata file.
The $COOKBOOK is also not needed in the chefspec step and will be removed to
help simplify these steps to only what is necessary.

Change-Id: I90ef4b60fb8a9706cd3670cb2d4bdb06ed4b3d8e
Closes-Bug: #1370210
Merge "Remove unnecessary metadata check"