bootstrap: allow using alternate remotes
bootstrap: allow using alternate remotes

If you want to bootstrap from a fork, the `--remote` option may be used
to select it.

Also limit the branches to 'develop' and 'master' if the remote is
'origin' since those are the actual integration branches used (other
branches on 'origin' are just PR branches).

tmerrick1/spack,LLNL/spack,tmerrick1/spack,tmerrick1/spack,TheTimmy/spack,TheTimmy/spack,lgarren/spack,matthiasdiener/spack,iulian787/spack,EmreAtes/spack,krafczyk/spack,krafczyk/spack,matthiasdiener/spack,krafczyk/spack,TheTimmy/spack,LLNL/spack,lgarren/spack,LLNL/spack,skosukhin/spack,iulian787/spack,mfherbst/spack,matthiasdiener/spack,lgarren/spack,mfherbst/spack,TheTimmy/spack,skosukhin/spack,krafczyk/spack,krafczyk/spack,iulian787/spack,skosukhin/spack,lgarren/spack,iulian787/spack,tmerrick1/spack,LLNL/spack,matthiasdiener/spack,iulian787/spack,TheTimmy/spack,EmreAtes/spack,skosukhin/spack,skosukhin/spack,mfherbst/spack,LLNL/spack,EmreAtes/spack,matthiasdiener/spack,EmreAtes/spack,lgarren/spack,mfherbst/spack,tmerrick1/spack,mfherbst/spack,EmreAtes/spack