environs/httpstorage: test HEAD Location URL
environs/httpstorage: test HEAD Location URL

Actually try querying the URL, to make sure it's
valid for the storage.

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