- Copy stable/10@296371 to releng/10.3 in preparation for 10.3-RC1
builds.
- Update newvers.sh to reflect RC1.
- Update __FreeBSD_version to reflect 10.3.
- Update default pkg(8) configuration to use the quarterly branch.

Approved by: re (implicit)

MFC r285722, r285733:

r285722 (brd):
Add support for building VirtualBox Vagrant images.
Abstract the build, package and upload to handle building
either type.

r285733
Fix an out-of-order execution issue regarding pkg(8):
- pkg(8) cannot be removed before subsequent reinvocations
- The PKG_CACHEDIR cannot be cleaned after the repo*.sqlite
has been removed
- pkg(8) cannot be removed as a precursor to any of the other
steps involved here

Approved by: re (kib)
Sponsored by: The FreeBSD Foundation

Add support for building VirtualBox Vagrant images.
Abstract the build, package and upload to handle building either type.

Approved by: re (gjb)