[r=menno.smits],[bug=1302005] cmd/juju: refactor --series to use StringsValue
[r=menno.smits],[bug=1302005] cmd/juju: refactor --series to use StringsValue

seriesVar is now called seriesValue and is implemented using
StringsValue. This reuses StringValue while preserving the series
validation logic.

https://codereview.appspot.com/94060044/

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