add cluster subcommand, and combine cluster start and bcbio bootstrap

instead of:

  bcbio_vm.py elasticluster start bcbio
  bcbio_vm.py aws bcbio bootstrap

you can now simply:

  bcbio_vm.py aws cluster start

other commands that have changed:

bcbio_vm.py elasticluster ssh bcbio -> bcbio_vm.py aws cluster ssh
bcbio_vm.py elasticluster stop bcbio -> bcbio_vm.py aws cluster start
bcbio_vm.py aws bcbio run -> bcbio_vm.py aws cluster command
bcbio_vm.py aws bcbio bootstrap -> bcbio_vm.py aws cluster bootstrap