Update install command to run any scripts

Run a script is parallel on many machine. The script can be either local or
remote. The script is written and then executed.

  lttng-cluster -H localhost -u ubuntu install runscript myscript.sh

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Refactoring the install command

The install command is now only a hook to copy a shell script and execute it.
It allows to execute any script, either from localhost, or on a remote host
with http. Maybe this command should be renamed to reflect the new capability.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>