Add ability to delete root node with ytypes.DeleteNode

[DanG100]

If len(path.Elem) == 0, then the DeleteNode should clear it.

[coveralls]

Coverage increased (+0.005%) to 90.842% when pulling 3082053d1bcbe7f3d7f7cc34e0e665ad6a6dac22 on DanG100:delete-root into c15884d06738b435020d84e167f7143627bf19ea on openconfig:master.

[wenovus]

Could you comment on what use case you're intending to use this for? I'm thinking this has to do with the fakeroot but I think understanding the use case will make it easier to judge whether this is an appropriate solution.

[wenovus]

Incoporated in https://github.com/openconfig/ygot/pull/904