(QENG-1126) Allow setting IS_PE to the negative

Previously we had an environment variable "IS_PE" or "BEAKER_IS_PE" to
set whether or not we were using PE. Since then PE has become the
default. Module authors would like a similarly simple way via the ENV
to override the default and set the installation type as "FOSS".
The convention that has arisen is to use the negative in the IS_PE
environment variable (e.g. "IS_PE=no"). This brings that convention
into Beaker proper.

Original work done by @cyberious (Travis Fields).
The prior PR would drop through with a `nil` value. Is this no longer a valid case?