Source node chef name from hostname per @jrosenboom puppetlabs/razor-server#250
Fix `razor-admin` when run from different directory

If the directory where a user runs the `razor-admin` command
contains its own Gemfile, that Gemfile was being used instead
of razor-server's. This script will now change directories into
the razor-server directory in code.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-356