Check for a running process should be more fine-grained

[mr-cal]

What needs to get done

Improve the granularity of the check that checks if the process that launched an instance is still active.

Currently, craft-providers will only check that the PID that created an instance exists. This could be improved by checking that /proc/<pid>/cmdline is a python process.

Suggested by @syu-w in #464 (comment)

Why it needs to get done

If the PID that launched the instance is currently in-use by another process, it may add a 60 second delay before the instance is auto-cleaned.

Consider a scenario where craft-providers created a base instance with pid 100, but the instance was not fully setup and pid 100 is now assigned to another active process (due to a reboot or the pid counter being reset).

Current behavior

1. Craft-providers sees the base instance is not ready and pid 100 is active
2. Craft-providers waits for the instance to be ready. If it is not ready and the instance's timer hasn't changed in 60 seconds, then it auto-cleans the instance.

New behavior

1. Craft-providers sees the base instance is not ready and that pid 100 is active but not a python process.
2. Craft-providers immediately auto-cleans the instance.