[NEXUS-5870] Allow additional LDAP environment variables to be configured via system properties

As app context is exported to system properties, use can configure them via nexus.properties

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #115 from sonatype/NEXUS-5870-custom-ldap-env-vars

REVIEW: [NEXUS-5870] Allow additional LDAP environment variables to be configured via system properties