scm-source sanity check for mark trusted

[lotharschulz]

Marking an image trusted shall be only possible under these conditions:

• is scm-source info present
• is it syntactically correct, according to https://stups.readthedocs.io/en/latest/user-guide/application-development.html#scm-source-json
• is url a valid git url?
• is revision a full commit hash (not an abbreviated one)
• git status was clean (empty)

[lotharschulz]

https://github.com/zalando-stups/pierone-cli/issues/64#issuecomment-398010693 follow up

[juhmelo]

this is implemented on pierone side https://github.bus.zalan.do/automata/piertwo/pull/18