Add default remote_start/remote_exec/remote_put/remote_get/remove_finish for containers

In tor-browser-build/rbm.conf we define remote_start, remote_exec, remote_put, remote_get, remote_finish scripts to use the container script (which is included in rbm). I think instead of defining them in tor-browser-build we should define them in lib/RBM/DefaultConfig.pm, with an option to enable them, to make it easier to use outside of tor-browser-build.

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