The TooOldComputeService upgrade check currently produces a warning,
which may be missed if the upgrade process only checks the exit code of
the upgrade check command.
Since this can lead to Nova control services failing to start, make the
upgrade check a failure instead, so it results in a non-zero exit code.
Closes-Bug: #1956983
Change-Id: Ia3ce6a0b0b810667ac0a66502a43038fe43c5aed
Pierre Riteau