add port-forwarding helpers on the NetworkInterfaceDriver

[jluebbe]

Description When using a RemoteNetworkInterface, it is useful to open connections from the exporter to the DUT. We can make that easy by using the sshmanager we already have for managing access to the exporter.

Also fix some minor issues on the way.

Checklist

• [ ] Documentation for the feature
• [x] Tests for the feature
• [x] CHANGES.rst has been updated
• [x] PR has been tested

[codecov[bot]]

Codecov Report

Base: 62.1% // Head: 63.4% // Increases project coverage by +1.2% :tada:

Coverage data is based on head (e5eca50) compared to base (fb3efbb). Patch coverage: 71.2% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #967     +/-   ##
========================================
+ Coverage    62.1%   63.4%   +1.2%     
========================================
  Files         151     151             
  Lines       11364   11288     -76     
========================================
+ Hits         7066    7163     +97     
+ Misses       4298    4125    -173     

Impacted Files	Coverage Δ
labgrid/util/agentwrapper.py	96.8% <ø> (ø)
labgrid/driver/networkinterfacedriver.py	62.9% <46.1%> (-7.4%)	:arrow_down:
labgrid/target.py	91.8% <60.0%> (-0.6%)	:arrow_down:
labgrid/driver/sshdriver.py	57.8% <66.6%> (+0.1%)	:arrow_up:
labgrid/util/ssh.py	91.3% <93.1%> (-0.1%)	:arrow_down:
labgrid/driver/common.py	83.3% <100.0%> (+1.8%)	:arrow_up:
labgrid/driver/__init__.py	100.0% <0.0%> (ø)
labgrid/util/proxy.py	74.6% <0.0%> (+1.7%)	:arrow_up:
labgrid/driver/powerdriver.py	70.6% <0.0%> (+4.1%)	:arrow_up:
labgrid/remote/client.py	49.7% <0.0%> (+4.9%)	:arrow_up:
... and 3 more

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[Bastian-Krause]

PR looks good to me. Only the comment above needs to be addressed.