Move some logic from update() to BaseLdap

Move immutable attributes' check to BaseLdap

bp refactor-ldap-driver

Change-Id: If321cd689dab68a397f61216de569200a42ada5d
Move affirm_unique() in create() to BaseLdap

bp refactor-ldap-driver

Change-Id: I88cc07857f709f106c998b5d49c7772a6d012f9c
Merge "Move some logic from update() to BaseLdap"
Merge "Move affirm_unique() in create() to BaseLdap"