Add unique email contraint while creating user

[jashaik]

Version:

[Version of the project installed]

Environment:

As part of issue - #333, [email protected] and [email protected] are considered to be the same email addresses in user creation and should 409 conflict. But chef-zero is failing in this case

Scenario:

https://buildkite.com/chef/chef-chef-server-master-verify/builds/3936

Steps to Reproduce:

https://buildkite.com/chef/chef-chef-server-master-verify/builds/3936

Expected Result:

As part of issue - #333, [email protected] and [email protected] are considered to be the same email addresses in user creation and should 409 conflict.

Actual Result:

https://buildkite.com/chef/chef-chef-server-master-verify/builds/3936