Virtual docker cleanup

[stefanoborini]

Refactoring of the Virtual Docker to make it more extensible.
Introduced the VirtualDockerClient to behave like a normal dockerpy client.
Disambiguated many confusing names, and followed a more consistent approach to ids.

[codecov-io]

Codecov Report

Merging #389 into master will decrease coverage by 0.07%.
The diff coverage is 97.75%.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   95.24%   95.17%   -0.08%     
==========================================
  Files          94       94              
  Lines        3998     3996       -2     
  Branches      258      256       -2     
==========================================
- Hits         3808     3803       -5     
- Misses        133      139       +6     
+ Partials       57       54       -3

Impacted Files	Coverage Δ
remoteappmanager/docker/container_manager.py	82.48% <0%> (-3.69%)	⬇️
.../handlers/tests/test_register_container_handler.py	100% <100%> (ø)	⬆️
...ppmanager/docker/tests/test_async_docker_client.py	100% <100%> (ø)	⬆️
...eappmanager/docker/tests/test_container_manager.py	100% <100%> (ø)	⬆️
remoteappmanager/cli/tests/test_remoteappdb.py	100% <100%> (ø)	⬆️
remoteappmanager/tests/test_command_line_config.py	100% <100%> (ø)	⬆️
remoteappmanager/docker/tests/test_image.py	100% <100%> (ø)	⬆️
remoteappmanager/webapi/admin/tests/test_user.py	100% <100%> (ø)	⬆️
...eappmanager/webapi/admin/tests/test_application.py	100% <100%> (ø)	⬆️
...teappmanager/webapi/admin/tests/test_accounting.py	100% <100%> (ø)	⬆️
... and 16 more

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