Further checks and docs

[sebastian-luna-valero]

Summary

I prefer to revert https://github.com/EGI-Federation/fedcloud-vm-monitoring/commit/5759258141eb9e1251ef7e75248374e31fc47c46

Checking the name of VM images as proposed in this PR:

• is more readable for me
• is faster (checked)
• turned out to be more accurate in further tests

I am also adding some notes to the relevant function to clarify how it works.

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Related issue :