support multiple OUs in LDAP component

LDAPConfig class has been enhanced to support a new parameter
"subDNs" which is a collection of prefixes to add to "baseDN"
parameter.
LDAPUserComponent has been modified to take into account the new
parameters and search in a loop.
Setting multiple OUs enables the usage of users from different LDAP
Organization Units.

Change-Id: I22fc321b4938590d717784c7eb200f3aa8c1d206
Signed-off-by: Calin CRECEA <calin.crecea@sap.com>