TSC-101 Rename key() methods

As per the plan outlined in https://lists.opendaylight.org/pipermail/mdsal-dev/2018-March/001519.html we MD-SAL is delivering this item:

2) Replace org.opendaylight.yangtools.yang.binding.Identifiable.getKey()

The full details are in the email, bottom line for downstreams is:

The solution is to rename getKey() to key(). As all user-governed names in generated code is prefixed (with "is", "get" and similar), this will permanently solve this particular problem. Unfortunately it will also break all code which deals with getting/putting elements into keyed lists. The fix is a mechanical one and probably scriptable (but I am not much for writing scripts).