Removed obsolete code for installing server_redhat packages with explicit names.

I suspect this resulted from a merge accident. This code was originally removed
in the 2012-08-06 commit for "[COOK-1532] - specify packages with attributes",
but it reappeared in the 2012-11-05 commit for "Support for postgresql 9.0.x on
opensuse 11.4".

Clearly, the old code badly breaks the redhat family for postgresql-9.X, and
since it probably has no effect in opensuse, I just decided to remove it.
Run initdb for any redhat-derived platform version on first install

This should have been done in the 2012-11-02 commit to "[COOK-1844]
Make service name configurable." That change corrected the initdb
call with the version-specific service name, and it also removed
the comment about the code being not valid for 9.x. It should have
removed the "if node[postgresql][version].to_f < 9.0", too.

That is what I have done, to complete that fix.
Provide PGDG yum repo to install PostgreSQL 9.X on redhat-derived distros.

This is the same approach used for the PostgreSQL backport PPA repository.
(Note yum_pgdg_postgresql.rb is to redhat as ppa_pitti_postgresql.rb is to debian.)
Moved hash for looking up http://yum.postgresql.org links into an attribute.
Merge pull request #35 from DonorsChoose/pgdg_yum_repository

[COOK-2232] Provide PGDG yum repo to install postgresql 9.x on redhat-derived distributions