F #265: Improve START_SCRIPT
F #265: Improve START_SCRIPT

Scripts run by one-contexd will not be executed in a subshell - this
will fix the issue with the START_SCRIPT which would hold the boot if
any long-running process would originate in it (e.g. daemon/service).

User still will need to do something like this inside the START_SCRIPT
(or any other context script) otherwise SIGTERM will be sent:

    nohup service nginx start &

Signed-off-by: Petr Ospalý <fdcc599d7519675578af009a8bb163559e4430c3@opennebula.io>

OpenNebula/addon-context-linux,OpenNebula/addon-context-linux