better image table handling

[jscotka]

Because some distros are not known, check if distro is know, otherwise fallback and use hardcoded fedora20 as default one

[coveralls]

Coverage increased (+3.86%) to 91.65% when pulling 7ef981e on jscotka:image_table into ab10697 on simon3z:master.

[coveralls]

Coverage decreased (-0.67%) to 87.13% when pulling 109d89e on jscotka:image_table into ab10697 on simon3z:master.

[coveralls]

Coverage decreased (-0.67%) to 87.13% when pulling 2e41e51 on jscotka:image_table into ab10697 on simon3z:master.

[jscotka]

Hmm, I don't know how to deal with .tox and these things. I've tested it locally and it works well.