Cleanup files on service removal

User story:

PMM creates some files in /tmp folder (e.g. cert files or webConfigPlaceholder in /tmp/mongodb_exporter/agent_id/xxx) when user adds a service to monitoring, these files are not removed when service is removed from monitoring.  

Acceptance criteria:

All related text files passed to exporters/agents are deleted on service removal

Out of scope:

Suggested implementation:

How to test:

Add a service to monitoring and check which files were created
Remove the service and verify all files related to the service were cleaned up

Details: