multitenancy: Add config key `auth:usernames are emails = yes`

With this enabled usernames like "foo@bar.com" will be considered as the
full user's name. Without this enabled mapped username will be foo and
bar.com will be the mapped domain name.

OpenChange multitenancy feature depends on this.
Merge branch 'zentyal-3.5' into multitenancy

Conflicts:
	debian/patches/series
Merge pull request #4 from Zentyal/multitenancy

Add config key `auth:usernames are emails`