Fix 'networkmanager:' backend options for modem connections

[slyon]

Description

The COMMON_BACKEND_HANDLERS have been forgotten for modem connections apparently. Add them to allow the definition of the special networkmanager: mapping, used for NetworkManager integration. E.g. this networkmanager.uuid setting:

network:
  version: 2
  renderer: NetworkManager
  modems:
    mycon:
      auto-config: true
      networkmanager:
        uuid: b22d8f0f-3f34-46bd-ac28-801fa87f1eb6

We do not (yet) use that information (like uuid) in the current implementation. But reading YAML via NetworkManager will be broken if the networkmanager: mapping is not accepted.

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[codecov-io]

Codecov Report

Merging #179 (eaff8b1) into master (85ff2f5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #179   +/-   ##
=======================================
  Coverage   98.91%   98.91%           
=======================================
  Files          49       49           
  Lines        7351     7356    +5     
=======================================
+ Hits         7271     7276    +5     
  Misses         80       80           

Impacted Files	Coverage Δ
src/parse.c	100.00% <ø> (ø)
tests/generator/test_modems.py	100.00% <100.00%> (ø)

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[sil2100]

Ok, this looks good. I mean, we don't support openvswitch: for modems, but it doesn't really matter.