check drivers if they all can use/set isPublic property on MachineImage

[vladmunthiu]

Since MachineImage type can now mark a image if it's public or not, we should investigate if it's safe to remove using the tags to signal this. Spoke with backend team and they'll prefer to use API call rather than tags to check this.

[maksimov]

We should keep the tags for time being, but adding a test to verify discrepancy between the tag and the isPublic() return value is a very good idea.