Improve ncd status logging

Fixes #12.

In addition, restore the original formatting of ncd status line in `/var/log/ncm-cdispd` to avoid breaking existing parser for this file (I know that line is parsed to raise attention when a component failed).

Still work in progress, would like to add the list of failed components as described in #12.
+1 for `get_executable()`... In fact in another issue I advocated for this! 

As for the `execute_if_exists()` proposal, I'm wondering why it would not be the standard behaviour for `execute()`. Is there really a use case to executing a non executable command?

Probably better to open an issue in CAF for this discussion...