SSH connection: Ask user before connection to unknown host/add host to known_hosts.

[y0urself]

What:

• Ask user if he wants to connect to an unknown host
• Ask user if he wants to add unknown host to known_hosts
• Use RejectPolicy() instead of AutoAddPolicy()

Why:

• The paramiko Authetication policy AutoAddPolicy() is insecure because of Man in the middle attacks.

How:

• Try to build an equivalent to the openssh lib approach.

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #486 (569dcc4) into master (98b4601) will decrease coverage by 0.16%.
The diff coverage is 88.88%.

❗ Current head 569dcc4 differs from pull request most recent head 0724918. Consider uploading reports for the commit 0724918 to get more accurate results

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
- Coverage   98.98%   98.82%   -0.17%     
==========================================
  Files          54       54              
  Lines        3847     3908      +61     
  Branches      928      933       +5     
==========================================
+ Hits         3808     3862      +54     
- Misses         27       34       +7     
  Partials       12       12              

Impacted Files	Coverage Δ
gvm/connections.py	87.65% <88.88%> (+0.30%)	⬆️

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