DRY up test harness to use spec file for build deps

This makes it easier to assure all build deps are correct in spec.

* explicitly require rpmdevtools since it's a "hidden"/indirect build dep on el5 to build tito
* use yum-builddep to install requirements based on spec file (DRY dockerfiles)
* add rawhide to test harness for early detection of breaking changes

multiple commits: see commit messages for details

```
Summary
/tmp/titotest-centos-5.9-python.out     : OK (SKIP=2)
/tmp/titotest-centos-6.4-python.out     : OK
/tmp/titotest-fedora-20-python.out      : OK
/tmp/titotest-fedora-rawhide-python.out : OK     <--- new, adds 35 seconds
/tmp/titotest-fedora-20-python3.out     : OK
/tmp/titotest-fedora-rawhide-python3.out: OK     <--- new, adds 35 seconds
```

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todo in the future when centos7 becomes available as docker base image: add c7 to test harness