do not attempt to re-create unvalidated image servers with .from_url

closes #968 (closed)

requested review from @erwanrouchet

assigned to @mlbonhomme

added 5 commits

• 7a051636...95eb85e0 - 4 commits from branch master
• 5803fc00 - avoid re-creating un-validated image servers + test

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resolved all threads

added 3 commits

• 872d2145 - 1 commit from branch master
• c5124560 - avoid re-creating un-validated image servers + test
• d988f187 - remove useless elif

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mentioned in commit 4c8d3d9c

merged