BUG-1092: rename data.api.InstanceIdentifier to YangInstanceIdentifier

On popular request to clear up confusion between
yang.binding.InstanceIdentifier and yang.data.api.InstanceIdentifier, we
rename the latter (which is the low-level one). This will make it clear
for users of GUIs, who were confused by the tools giving them the choice
between the two.

Change-Id: I1e30a0df93d8bfb891d047c58abcb117d4006c76
Signed-off-by: Robert Varga <rovarga@cisco.com>