Don't treat optional requirements as mandatory.

Fixes issue where optional requirements from a Felix OBR repository were treated as mandatory.

git-svn-id: https://svn.apache.org/repos/asf/aries/trunk@1449267 13f79535-47bb-0310-9956-ffa450edef68
Add tests to verify optional requirements are not treated as mandatory.

git-svn-id: https://svn.apache.org/repos/asf/aries/trunk@1449268 13f79535-47bb-0310-9956-ffa450edef68