EXT: NEXUS-6582 - Yum Generate Metadata does not detect new rpms added on filesystem

Hi,

This is my first time submitting a pull request for nexus-oss so apologies if i have did something wrong.

This change adds a checkbox to the Yum Generate Metadata Task Configure screen to allow you to force a full scan and not use the previously generated RPM list file.  This will allow the Yum Generate Metadata task to pick up new RPMs which have been added directly to the filesystem

thanks
Geoff