Add <targets> element to <start_scan>.

[jjnicola]

It is possible now to specify different ports for different targets,
and run them all from the same task.
Each target, which can be a multi-host target, may have its own port list.
For this reason, get_scan_ports() receive the optional parameter 'target',
to return the corresponding port list for this target. In other case it
returns the port list of the first target for all targets.

For backward compatibility purposes it is still allow to pass a
target list and a port list as attributes, in which case the
element will be ignored. As before, exec_scan is called for each host
and the port list is the same for each host.

Document in start_scan and add example. Document compatibility changes
Add unittest.

[codecov]

Codecov Report

Merging #34 into master will increase coverage by 2.62%.
The diff coverage is 58%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   60.44%   63.06%   +2.62%     
==========================================
  Files           4        4              
  Lines        1082     1102      +20     
==========================================
+ Hits          654      695      +41     
+ Misses        428      407      -21

Impacted Files	Coverage Δ
ospd/misc.py	59.95% <44.44%> (+5.42%)	⬆️
ospd/ospd.py	67.9% <60.97%> (+0.9%)	⬆️

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