161 improve verbose mode

[daniel-santos]

Checklist

• [not fully] 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 #161

Description of Changes

enabling verbose_mode now causes curl command and payload to be dumped to syslog via stderr/stdout