Initialize end_time with create_scan

[jjnicola]

What:
Initialize end_time with create_scan

Why:
Some methods assume end_time is set.
Close issue greenbone/ospd-openvas#361

How:
Set the option scaninfo_store_time and start a scan. Without this patch, the scan crashed.

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #354 (15b5ba6) into ospd-20.08 (60c6166) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           ospd-20.08     #354      +/-   ##
==============================================
+ Coverage       73.90%   73.91%   +0.01%     
==============================================
  Files              23       23              
  Lines            2602     2603       +1     
==============================================
+ Hits             1923     1924       +1     
  Misses            679      679              

Impacted Files	Coverage Δ
ospd/scan.py	92.56% <100.00%> (+0.03%)	⬆️

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