Add optional keyfile argument to rake tasks

This addition walks over any extra arguments provided to the
rake task. If the file is a key file, it is used to set the
BEAKER_keyfile ENVIRONMENT variable for beaker-rspec and/or
the `--keyfile` command line argument for beaker as needed.

Example:

BEAKER_setfile=../vcenterhost.cfg rake beaker:rspec:test[foo,pe,'/home/myuser/.ssh/id_rsa-secret']