morph exec should have a --script option

[johanot]

It should be possible to supply a full shell script to morph to be executed on a/multiple remote host(s).

Something along the lines of morph exec --script foobar.sh ./hostgroups/k8s.nix

The supplied script would be uploaded to the remote hosts as tempfiles and executed like so: bash /tmp/xXya81.foobar.sh

If the --script flag is provided, the command argument should either be:

• omitted

• its value be prepended to the executing shell command at the remote

• its value be replacing the executing shell command at the remote