Copy the contrib directory instead of moving it

This is necessary because pbr is stupid enough not to be
able to handle symlinks. The initial symlink was necessary to
avoid breaking the functional job. Once change [1] merges we
can drop the old contrib directory.

[1] https://review.openstack.org/#/c/142603/

Change-Id: I683009b1c5b3520e5f9f2c17a9a0dabefdeebe45