(QENG-923) Ensure @codename is set if platform string has codename

Beaker 1.14.0 and up has a change to Beaker::Platform which ensures that
@version and @codename is set if the platform string contains the
version number, and that @version is set if platform string contains the
codename, but it fails to set the @codename in this later case.

Puppet currently has debian/ubuntu configs checked (used in ci) which
use platform strings of the form debian-wheezy-x86_64. We could change
these, but it looks like the intention of the Beaker::Platform object is
to provide @version and @codename (where codenames are used).

This patch ensures that @codename is set along with @version in these
cases.