create a subcommand

Because we might want to specify *what* repo we want to install
remotely (cephdeploy.conf can have a bunch listed)

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
update the docstring for install_repo

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>