security manager: change default email for auth_remote_user
security manager: change default email for auth_remote_user

The ab_user SQL table may have a unique constraint for the
'email' attribute, leading to failures while inserting data
for a new user registration. Changing the default to a
unique value should fix the problem.

Issue: #965

dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder