Fix ldap stored homedir

Stop saving the homedir as user setting as it undermines configurability with datadir and/or the LDAP homedir setting. This may also fix https://github.com/owncloud/core/issues/1450

Also fixes save and get for this option in LDAP settings.

Also adds a userExists check in OC_User to make sure that the responsible backends will be called.

Reviewers please @icewind1991 @schiesbn @georgehrke @karlitschek or others