Add OSP command get_vts.

[janowagner]

This adds the command get_vts, for getting the list and details about
vulnerability tests a OSP scanner might offer.

So far, the tests have only an id and a name. It is intended
to extend this by some standard elements which are shared among
different scanners and with individual elements which are very specific
for a scanner.

With this status, it would be possible to have a client get the list and
allow the user to select one or many tests that are desired.

Follow-ups will be to add receive the vt selection from the client in
order to execute only these.

This change uses a simple dict for storage. With extending
capabilities, it might be helpful to use advanced containers/collections.

• ospd/ospd.py: Add command get_vts and handling of data. Increase version
  of OSP to 1.2.

• doc/OSP.xml: Add documentation for command get_vts.

[codecov]

Codecov Report

Merging #12 into master will increase coverage by 1.03%.
The diff coverage is 82.5%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage    54.5%   55.54%   +1.03%     
==========================================
  Files           4        4              
  Lines         998     1037      +39     
==========================================
+ Hits          544      576      +32     
- Misses        454      461       +7

Impacted Files	Coverage Δ
ospd/ospd.py	61.49% <82.5%> (+1.53%)	⬆️

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