Add the nightly build target to builder.py.

If I understand this correctly, this PR moves the management of the nightly target into builder.py which is great. I don't see this blacklist behavior present in builder.py which I expected to find so that we don't build pulp-server on EL5. I thought that pulp-server wouldn't even be built for EL5, but it seems that it is based on [line 48 in builder.py](https://github.com/pulp/pulp/blob/rbarlow-nightly_target/rel-eng/builder.py#L48). Do you think this is right?