Support pkg build for Managed Rel URLs, add tests

Support pkg build for Managed Rel URLs, add tests

The new Managed Release Common Distribution, the distro that contains
both Managed and Self-Managed projects that we now bless as our official
releases (starting with Fluorine), uses a new distro name prefix.

The packaging build logic had support for using the correct distro name
prefix based on Karaf 3 or 4 version. Extend that logic to optionally,
but with priority, return the distro name prefix based on a download
URL. Include checks to cover MR URLs, thereby adding MR URL support.

Also add unittest coverage that finds this failure.

Jira: INTPAK-203
Change-Id: Ie1180fb78098eac6758224e3f166f6e0d1f8b629
Signed-off-by: Daniel Farrell <dfarrell@redhat.com>