Fix incorrect NotImplementedError

neutron.common.exceptions doesn't define NotImplementedError, but
some codes still use it. We should use builtin
exceptions.NotImplementedError instead.

Note: this patch also fixes a never run code in nvp.

Closes-Bug: #1221500
Change-Id: I5f367ab5edc1e7fbbc2e4eba5fe36d148d4d062d
Merge "Fix incorrect NotImplementedError"