SEC-2718: use container's ClassLoader in LdifFileLoader

By setting the container's ClassLoader in the constructor of the
LdifFileLoader it is possible to load LDIF files from the classpath
of an executable WAR file.

This ensures that an embedded LDAP server can load a LDIF file from a
WAR file, whether it is executed or deployed into an external servlet
container.

JIRA issue: [SEC-2718](https://jira.spring.io/browse/SEC-2718)

CLA submitted.