Gracefully handle Users with no email attribute

In certain environments, such as when using Keystone with a LDAP
backend, it's possible that the email attribute won't exist for all
users.

Change-Id: I71041be832206e52bfb73926199f2c89e0e84a0b
Closes-Bug: #1260423
Merge "Gracefully handle Users with no email attribute" into stable/havana