add rpmdevtools as build dep for el5

9feb9cf4 added rpmdevtools as an install depedency;
it is also needed as a build dep for current functional tests.
DRY up test harness to use spec file for build deps

Ensure build deps are consistent across the spec file and test harness.

Note: http://fedoraproject.org/wiki/HOWTOFindMissingBuildRequires
says that some packages should not be BuildRequires because
"they would occur too often" and "are considered the minimum build
environment." However, Docker changes the scope of that assumption
to keep docker images small.
add f19 to test harness to test conditional build deps

* fedora 19 does not have rpm-python3, thus we only test python2.
* assert that build deps are correct for fedora 19
use "semi-official" f20 base instead of mattdm/fedora

https://index.docker.io/_/fedora/ states:

> This image is to be considered semi-official from fedora's side
> (until approved by fedora rel-eng)
add fedora-rawhide to test harness

Use the "semi-official" base image for rawhide.
It appears to be updated on a regular basis.

This gives us advance warning of breaking changes
in the bleeding-edge version of fedora.
remove f19 from test harness to save space and time

* Save disk space on dev host by avoiding `docker pull mattdm/fedora'
* Save time by not building a fedora 19 image and running its tests

We can always revert this commit if we need to run tests on f19.
Merge pull request #135 from jumanjiman/dry

DRY up test harness to use spec file for build deps