utils/ssh: revert to using slices for command args
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

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