A lot of CI code is python mixed with bash, e.g.,

    ssh = getConnection(host)
    ssh.run('ls')
    ssh.run('echo')

At least some of this CI code would benefit if it was written in a
simple bash script, returning error codes and potentially other
information either through stdout/stderr or files, to the calling Python
code:

    ssh = runScript(host, script)
    # script does: ls; echo

This commit introduces the possibility to run entire scripts. The idea
is that the executor has a script (on localhost), which is either
executed locally or on a remote host. For the remote host, the script is
not copied but piped into a remotely executed bash. In both cases,
output is either returned like the Cmd.run() function with returncode
and mixed stdout/stderr, or optionally redirected into a file on the
(remote) host, which can be treated further by the Python code in later
steps.