[r=axwalk],[bug=1298115] utils/ssh: revert to using slices for command args
[r=axwalk],[bug=1298115] utils/ssh: revert to using slices for command args

Reverts part of https://codereview.appspot.com/66340045/
which changed OpenSSH-based client args from using a slice
to a map.

Fixes lp:1298115

https://codereview.appspot.com/81180043/
mjs/juju,mhilton/juju-juju,davecheney/juju,mjs/juju,mattyw/juju,waigani/juju,frankban/juju-tmp,axw/juju,dimitern/juju,marcmolla/juju,fwereade/juju,sinzui/juju,bogdanteleaga/juju,mjs/juju,rogpeppe/juju,mhilton/juju,dimitern/juju,axw/juju,axw/juju,makyo/juju,anastasiamac/juju,fwereade/juju,cherylj/juju,mjs/juju,kat-co/juju,frankban/juju-tmp,axw/juju,voidspace/juju,perrito666/juju,cloud-green/juju,mhilton/juju-juju,dooferlad/juju,mattyw/juju,mikemccracken/juju,marcoceppi/juju,alesstimec/juju,mikemccracken/juju,fwereade/juju,marcmolla/juju,AdamIsrael/juju,tsakas/juju,dooferlad/juju,bac/juju,davecheney/juju,rogpeppe/juju,sinzui/juju,jrwren/juju,frobware/juju,Altoros/juju-cloudsigma,davecheney/juju,frobware/juju,bac/juju,wwitzel3/juju,bz2/juju,tsakas/juju,tsakas/juju,mhilton/juju,frankban/juju-tmp,thomnico/juju,Altoros/juju-vmware,axw/juju,howbazaar/juju,gabriel-samfira/juju,thomnico/juju,mjs/juju,wwitzel3/juju,frankban/juju,ericsnowcurrently/juju,tasdomas/juju,waigani/juju,anastasiamac/juju,anastasiamac/juju,macgreagoir/juju,reedobrien/juju,marcoceppi/juju,macgreagoir/juju,macgreagoir/juju,perrito666/juju,mikemccracken/juju,macgreagoir/juju,frankban/juju,reedobrien/juju,makyo/juju,cmars/juju,makyo/juju,makyo/juju,mhilton/juju,marcmolla/juju,ericsnowcurrently/juju,cherylj/juju,mhilton/juju-juju,gabriel-samfira/juju,marcmolla/juju,kat-co/juju,tasdomas/juju,bac/juju,bz2/juju,cmars/juju,jrwren/juju,mhilton/juju-juju,waigani/juju,frankban/juju-tmp,alesstimec/juju,marcoceppi/juju,wwitzel3/juju,voidspace/juju,howbazaar/juju,bac/juju,anastasiamac/juju,gabriel-samfira/juju,tasdomas/juju,bogdanteleaga/juju,bz2/juju,reedobrien/juju,mikemccracken/juju,kat-co/juju,fwereade/juju,perrito666/juju,waigani/juju,bogdanteleaga/juju,Altoros/juju-cloudsigma,bz2/juju,gabriel-samfira/juju,cmars/juju,frankban/juju,alesstimec/juju,howbazaar/juju,anastasiamac/juju,frobware/juju,bac/juju,frankban/juju,bz2/juju,tych0/juju,ericsnowcurrently/juju,AdamIsrael/juju,perrito666/juju,cloud-green/juju,wwitzel3/juju,thomnico/juju,sinzui/juju,Altoros/juju-cloudsigma,howbazaar/juju,AdamIsrael/juju,gabriel-samfira/juju,cloud-green/juju,mikemccracken/juju,reedobrien/juju,tasdomas/juju,thomnico/juju,Altoros/juju-vmware,voidspace/juju,cherylj/juju,perrito666/juju,dooferlad/juju,tych0/juju,jrwren/juju,tych0/juju,AdamIsrael/juju,voidspace/juju,rogpeppe/juju,mjs/juju,marcmolla/juju,ericsnowcurrently/juju,dimitern/juju,cloud-green/juju,cherylj/juju,reedobrien/juju,tych0/juju,sinzui/juju,mjs/juju,dooferlad/juju,mhilton/juju,cmars/juju,ericsnowcurrently/juju,mattyw/juju,bogdanteleaga/juju,dimitern/juju,macgreagoir/juju,marcoceppi/juju,tsakas/juju,mattyw/juju,dooferlad/juju,Altoros/juju-vmware,kat-co/juju,davecheney/juju,alesstimec/juju,Altoros/juju-vmware,Altoros/juju-cloudsigma,alesstimec/juju,dimitern/juju,howbazaar/juju,frankban/juju,kat-co/juju