We've already created the tags and targets for the 'split upcoming' repos 3.5-upcoming and 3.6-upcoming; we do not have to wait until 3.6 to start using them.

Use the following migration plan:
1. :check_mark: @Mat Selmeci releases osg-build 1.18.0
2. :check_mark: @Carl Edquist changes the mash config on repo-itb to pull from the osg-3.5-upcoming-* tags instead of the osg-upcoming-* tags and reverses the 3.5-upcoming -> upcoming symlink
3. :check_mark: we tag every build in osg-upcoming-* into the corresponding osg-3.5-upcoming-* tags
4. :check_mark: we check repo-itb to make sure it's been correctly populated
5. :check_mark: we change the osg-upcoming-elX targets to osg-3.5-upcoming-elX-build -> osg-3.5-upcoming-elX-development (i.e. be the same as the osg-3.5-upcoming-elX targets)
6. :check_mark: lock the old osg-upcoming-* tags so people can't build or promote into them
7. :check_mark: check repo-itb again
8. :check_mark: make the same changes on repo
9. :check_mark: Tell people to upgrade to osg-build 1.18.0 and use --3.5-upcoming instead of --upcoming
10. :check_mark: cd branches/3.5-upcoming; osg-build koji --3.5-upcoming *