USe `trueorfalse` for INSTALL_TESTONLY_PACKAGES

INSTALL_TESTONLY_PACKAGES is assumed to be set to `True` or `False`.
However, in devstack-gate this variable is set to 0 or 1. The patch uses
the already existing `trueorfalse` function to evaluate
INSTALL_TESTONLY_PACKAGES and normalize its value.

Change-Id: I0e4a31e422bad2a31d919d9f871e24833a0faa99
Merge "USe `trueorfalse` for INSTALL_TESTONLY_PACKAGES"