stevedore no longer raises a KeyError when there's an entrypoint
missing. Instead we must supply a callback that handles that error.
Update inspection code to work with this.
Also bumps stevedore minimum to 1.16 ahead of the global-requirements
bot.
Closes-Bug: #1603542
Change-Id: I12af23f2525ac90e577bdd10bbfbbd9788e9551c
Depends-On: I8aa1ee52ff7de50488acb86e8920da89ddb05771