Empty email for users gets saved as an empty string

The actual behavior is that horizon sends an empty string
on the "email" field when creating a new user if the field
isn't filled by the user.
To match the keystone client behaviour, instead of sending
an empty string, this patch sends a "None" value when the
field isn't filled by the user.
By doing this, when creating a new user from both,
keystone client and horizon, a "null" value is stored
under the same conditions.
in the database. This will keep the data consistency.
From the presentation point of view, when a "null" value
is received, it is presented as an empty string instead
of showing "None" on the "email" field of the users table.

Change-Id: Ib5df774b3fe9d82c084f380c9c11aea46b2984aa
Closes-Bug: #1265805
Merge "Empty email for users gets saved as an empty string"