Use rhsm.baseurl for ostree urls as well.

Extract the url join used by repolib into
utils.url_base_join. Add some tests.

Update repolib and ostree/config.py to use.

Note this waits till the last minute to right out
the repo urls as full urls.
Merge pull request #981 from candlepin/alikins/ostree_just_baseurl

Use rhsm.baseurl for ostree urls as well.