RKE should allow users to specify their own Kubernetes images

[sheng-liang]

Users should be able to install, for example, standard hyperkube images with RKE. We need to make sure everything (like host name etc.) is still setup correctly in that case.

[moelsayed]

At this point, we have tested both Rancher Kubernetes images and Hyperkube images. Both work with no issues.

In theory, any Kubernetes image, with Kubernetes binary installed in $PATH should work fine with no problems since we don't do or use anything specific to the image.

[sheng-liang]

Great. I'll close the issue if we don't have anything that depends on the content of the Kubernetes image.

[galal-hussein]

@sheng-liang Yes we don't have any dependencies on the content of the image except for the binaries to be in the PATH