MT#6561 Make sure daemon executable is available before executing any actions

Otherwise we might run into situations like that:

| Removing ngcp-schema ...
| Force-reloading ngcp-panel to handle schema changes
| [....] Restarting web application: ngcp-panel not runningstart-stop-daemon: unable to stat /usr/share/ngcp-panel/ngcp_panel_fastcgi.pl (No such file or directory)
| [FAIL] error ... failed!
|  failed!
| invoke-rc.d: initscript ngcp-panel, action "force-reload" failed.