Remove obsolete note from ldap

There was a note in the ldap assignment backend about normalizing
a DN using .upper(). This note is obsolete with the recent change
to use a DN compare function.

Change-Id: I34c69573498b5f2d2103ea485160df421c6f2b03
Related-Bug: #1309228
Code which gets and deletes elements of tree was moved to one method

Code which uses 'search_s' and 'delete_s' in ldap backends was moved
to a new method to remove duplication of code.

Added handling of exception ldap.NO_SUCH_OBJECT when we couldn't
find a search base while deleting. Now this exception is
not ignored. Instead of this raised NotFound exception
or more suitable (RoleNotFound, etc).

Also added handling of situation when we trying to delete
nonexisting entries.

Closes-Bug: #1316128
Change-Id: I8e7402337ab4d75ce8920e8158a6e1dd1ed97abd
Merge "Code which gets and deletes elements of tree was moved to one method"
Merge "Remove obsolete note from ldap"