v3 endpoint create should require url

The url parameter of endpoint object is not nullable in db, but we
do not have a null check in api. So, when create a endpoint if I
do not specify url parameter I will get a 500 reponse.
This patch will add an attribute require check to url parameter.

Change-Id: I9c8aa3bcb99ea5cb5e4e4d3306d8d222c6f074d1
Closes-Bug: #1284422
Merge "v3 endpoint create should require url"