REVIEW NEXUS-6591: Open up LDAP plugin

Changes:
* SSL plugin OSSed
* LDAP plugin OSSEd
* "legacy" OSS LDAP plugin removed
* common-ldap module imploded into ldap plugin

Packages are still left as "com." but GAVs
adjusted to "org."

Issue
https://issues.sonatype.org/browse/NEXUS-6591

CI
http://bamboo.s/browse/NX-OSSF101