[Fix] Send background VPN errors and recovery as generic_message noti…

[stktyagi]

…fications #1049

Modified tasks and tasks zerotier consistently with utils. Tested via manual testing.

Fixes #1049

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #1049

Description of Changes

Extended config/utils.py for sending api tasks' notification with fixing trigger_vpn_server_endpoint to send generic message notification for error and recovery.

[coveralls]

coverage: 98.609% (+0.002%) from 98.607% when pulling 9f31948bd2260c72549ae3ce980d07d3c2e24f4a on issues/1049-send-generic-message into 0235ae5cd1fa3d2029658d703cadb4812d300066 on master.