setDN() uses a boolean flag instead of string.

This changes the method declaration to accept a boolean instead
of the new DN string. The change happens on the "experimenter"
table ("ldap" column) and not on "password".
SqlAction.java: Remove unused actions, add new.

This removes actions that relied on the "dn" column of the
"password" table and also add surrogate actions that make use
of the new "ldap" column.
PasswordUtil.java: Use updated SQL action.

This simplifies the usage of PasswordUtil and the logic in AdminImpl
and ResetPasswordRequestI.
LdapImpl.java: Update method logic.

This is due to the updates to SQL actions and also the now
available new direct way of checking if a user came from an LDAP
server.
LdapImpl.java: Mimic SQL action return values.

This hard-codes the column names, as they are expected by
AdminI. It is not a portable solution and could be done better.
Deprecate bin/omero ldap discover.

Since the DN can't get out of sync anymore, this method isn't
required.
PasswordTest.java: Fix failing unit test.