Prevent multiple trento agents from running on the same host

[rtorrero]

In PR #198 we added some code to store the trento agent version on the host metadata.

Thinking about this, I decided to check if a potential scenario where during an update somehow something fails to stop the old trento process and it stays running and a new one is started. This could mean that each process could report a different version, making it harder for the user to identify potential old versions of the trento agent running on hosts.

My quick test revealed that it's possible to have multiple trento agents running on the same host which is probably something we should prevent, e.g. by using a lockfile as suggested by @angelabriel