CFY-5147 Use psutil to stop the diamond process
CFY-5147 Use psutil to stop the diamond process

Instead of polling the PID ourselves, we can use psutil for that,
as it uses a more resilient method: its .is_running() function
looks up if the PID is still reported by the OS, and if it is,
compares the process creation time.

This change makes the stop_diamond function correctly notice that it
is stopped, even if the PID was reused by the OS.

cloudify-cosmo/cloudify-diamond-plugin,cloudify-cosmo/cloudify-diamond-plugin