Bug 29744: (QA follow-up) Only consider PLACK_ENV for underscore

Bug 29744: (QA follow-up) Only consider PLACK_ENV for underscore

When run under certain circumstances (in jenkins, for example) some ENV
variables are set for convenience, like PLACK_WORKERS and
PLACK_MAX_REQUESTS and is causing some tests to fail (notably, shib
ones).

This patch makes the regex only consider PLACK_ENV when testing for the
underscore case.

Tests are updated accordingly, and they are also rewritten to test for
boolean values instead of empty string, or zero or one, etc. The
implementation shouldn't matter as long as the boolean evaluation is
correct and it is clearer for devs what to expect.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>