[r=jameinel] cmd/juju/destroymachine: compatibility with 1.16
[r=jameinel] cmd/juju/destroymachine: compatibility with 1.16

Merges the fix for bug #1253643 from 1.16 into trunk.

This updates "juju destroy-machine" so that if the API is not available, we fall back to the old 1.16 code path (directly accessing the database).

https://codereview.appspot.com/33780043
sinzui/juju,waigani/juju,ericsnowcurrently/juju,mhilton/juju-juju,sinzui/juju,perrito666/juju,mjs/juju,cloud-green/juju,mhilton/juju-juju,frobware/juju,frankban/juju-tmp,reedobrien/juju,mikemccracken/juju,cherylj/juju,marcoceppi/juju,perrito666/juju,jrwren/juju,thomnico/juju,voidspace/juju,marcoceppi/juju,gabriel-samfira/juju,Altoros/juju-cloudsigma,bz2/juju,voidspace/juju,frankban/juju,bz2/juju,rogpeppe/juju,voidspace/juju,fwereade/juju,jrwren/juju,tych0/juju,anastasiamac/juju,bz2/juju,bac/juju,cherylj/juju,axw/juju,dimitern/juju,marcmolla/juju,anastasiamac/juju,anastasiamac/juju,Altoros/juju-vmware,mattyw/juju,tsakas/juju,reedobrien/juju,dimitern/juju,wwitzel3/juju,cloud-green/juju,waigani/juju,frankban/juju,waigani/juju,fwereade/juju,jrwren/juju,rogpeppe/juju,kat-co/juju,howbazaar/juju,gabriel-samfira/juju,fwereade/juju,wwitzel3/juju,gabriel-samfira/juju,alesstimec/juju,mjs/juju,dimitern/juju,AdamIsrael/juju,mhilton/juju-juju,cloud-green/juju,dooferlad/juju,Altoros/juju-vmware,sinzui/juju,mhilton/juju,davecheney/juju,reedobrien/juju,AdamIsrael/juju,wwitzel3/juju,mattyw/juju,fwereade/juju,frobware/juju,davecheney/juju,anastasiamac/juju,dimitern/juju,macgreagoir/juju,alesstimec/juju,cmars/juju,bogdanteleaga/juju,AdamIsrael/juju,howbazaar/juju,makyo/juju,AdamIsrael/juju,davecheney/juju,macgreagoir/juju,wwitzel3/juju,mjs/juju,thomnico/juju,howbazaar/juju,frankban/juju,bogdanteleaga/juju,alesstimec/juju,alesstimec/juju,howbazaar/juju,marcoceppi/juju,tych0/juju,rogpeppe/juju,macgreagoir/juju,bz2/juju,tsakas/juju,mjs/juju,waigani/juju,dooferlad/juju,tasdomas/juju,macgreagoir/juju,ericsnowcurrently/juju,kat-co/juju,anastasiamac/juju,axw/juju,mattyw/juju,Altoros/juju-cloudsigma,bac/juju,thomnico/juju,alesstimec/juju,Altoros/juju-cloudsigma,mikemccracken/juju,dimitern/juju,tasdomas/juju,marcmolla/juju,cherylj/juju,perrito666/juju,bac/juju,marcmolla/juju,dooferlad/juju,davecheney/juju,cmars/juju,kat-co/juju,sinzui/juju,mattyw/juju,frankban/juju-tmp,reedobrien/store,axw/juju,mjs/juju,kat-co/juju,mhilton/juju,mhilton/juju,perrito666/juju,tasdomas/juju,Altoros/juju-vmware,voidspace/juju,bogdanteleaga/juju,ericsnowcurrently/juju,mikemccracken/juju,axw/juju,mjs/juju,mhilton/juju-juju,dooferlad/juju,reedobrien/juju,mikemccracken/juju,ericsnowcurrently/juju,frobware/juju,gabriel-samfira/juju,marcmolla/juju,mhilton/juju,howbazaar/juju,dooferlad/juju,mikemccracken/juju,tych0/juju,gabriel-samfira/juju,frankban/juju,frankban/juju-tmp,marcoceppi/juju,marcmolla/juju,reedobrien/juju,makyo/juju,tych0/juju,bogdanteleaga/juju,frankban/juju-tmp,kat-co/juju,cherylj/juju,tasdomas/juju,cmars/juju,makyo/juju,frankban/juju,axw/juju,Altoros/juju-vmware,thomnico/juju,makyo/juju,cmars/juju,bac/juju,bz2/juju,macgreagoir/juju,tsakas/juju,bac/juju,ericsnowcurrently/juju,mjs/juju,tsakas/juju,perrito666/juju,Altoros/juju-cloudsigma,cloud-green/juju