provider/null: fix intermittent test failure
provider/null: fix intermittent test failure

The ssh command line options changed, so we
were checking for the wrong command line.
Made it a bit less fragile by doing a negated
grep.

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