[feature] Allow not depending on rpcd-mod-iwinfo

[nemesifier]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• 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.

Description of Changes

On space constraint devices which do not have a WiFi card, it is useful to avoid including rpcd-mod-iwinfo and its dependencies.

[coveralls]

Pull Request Test Coverage Report for Build 10310291912

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 99.659%

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Files with Coverage Reduction	New Missed Lines	%
../files/sbin/netjson-monitoring.lua	3	97.93%

Totals
Change from base Build 10275642953:	-0.03%
Covered Lines:	3214
Relevant Lines:	3225

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💛 - Coveralls