Add config key `auth:usernames are emails`

With this enabled usernames like `foo@bar.com` will be considered as the full user's name (in the default domain). Without this enabled, mapped username would be `foo` and `bar.com` will be the mapped domain name.

OpenChange multitenancy feature depends on this.

If the config key doesn't exist (or is set to `no`) the behaviour of ntlm authentication is unchanged.