NewImage constructors should allow overriding default image properties

[micahyoung]

Right now, several image parameters get set by default on a new, empty image, which can only be overridden with Set*(). It would be convenient to be able to set these in the constructor.

The most useful constructor parameters for common image config properties would be:

• os - remote only, to override linux default
• os.version - local/remote, for setting compatible Windows kernel version.
• architecture - local/remote, to override amd64 default

This should also be considered along with #54 which will need a way to disambiguate manifest lists during FromBaseImage (also os, os.version, architecture, though these fields map to the manifest platform not the image config)

[natalieparellano]

@micahyoung was this resolved in #91 ?