Fix cmd line too long

[daniel-santos]

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 #159

Description of Changes

Uses --data's special treatment of @ to load data from file instead of in-lining it on the command line.