Bug 28385: ldap server configuration broken since migration from XML::Simple

Bug 28385: ldap server configuration broken since migration from XML::Simple

Bug 28278 removed usage of XML::Simple which had side-affect that
C4::Context->config("ldapserver") returns one more level of hash
based on id of element, so ldap configuration was no longer
available generating error in log:

No ldapserver "hostname" defined in KOHA_CONF: /etc/koha/sites/srvgit/koha-conf.xml

which is very confusing since hostname is there, but we are looking
at wrong place for it.

== Test plan ==
1. Setup an LDAP server with users or use a public one
     See https://wiki.koha-community.org/wiki/Ldap_testing
2. Configure Koha to use it, see example config above
3. Go to OPAC
4. Koha doesn't compile due to LDAP config not parsed correctly
5. Apply the patch
6. Koha compiles and page loads
7. Log in successfully with one of the LDAP users
8. Bonus: Try to log-in with a wrong username & a good password and
     vice-versa

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>