Fix bad pgrep / kill

[daniel-santos]

It's common to have forks of openwisp-monitoring due to use of $(some_command), so this can return more than one PID. Further, by calling kill "$pid" we just end up with an error about the parameter not being an integer.

Checklist

• [no] I have read the OpenWISP Contributing Guidelines.
• [yes] I have manually tested the changes proposed in this pull request.
• [n/a] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [n/a] I have updated the documentation.

Reference to Existing Issue

Closes #157

Please open a new issue if there isn't an existing issue yet.

Description of Changes

Only pick process whose parent is init, ignore others