OMERO5.1DEV__12: modify experimenter and password.

This commit drops the "dn" column from the "password" table
and adds update scripts for adding an "ldap" column to "experimenter"
and setting it to the correct value based on "dn".
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.
Update UI to make use of new "ldap" field.

This also fixes the way how the "LDAP" label was rendered
in the user account properties.
Make "ldap" column required.

This commit makes the column "not null" and updates all
the places where an experimenter was created explicitly with
the proper field initialization.
Update the behaviour of "ldap discover".

The following changes update "ldap discover" to again
provide a list of LDAP users which are disabled in the
OMERO DB:
- Revert "Deprecate bin/omero ldap discover."
  This reverts commit 742f54d52656fa4cb35c9f57b825e664c7b1cacc.
- Change parameter name in setDN().
- Change return type of discover() and add test.
- Minor fixes in ldap.py.
Merge pull request #3119 from bpindelski/2587_remove_dn

Remove the "dn" column from "password" (fix #2587).