update nginx::package to select the package class by $::osfamily

There were some bugs in the existing $::operatingsystem based approach.

* amazon was it's own package set when it's properly part of $::osfamily ==
'redhat' (only in recent factors)
* gentoo was improperly part of the amazon package set; this patch remove
support for gentoo but it was broken anyways

modifications to nginx::package::redhat were made as well

* it was trying to recover from $::lsbmajdistrelease not being defined -- this
was removed.
* the url to the nginx repo was including $::operatingsystem in it but
nginx.org only has package dirs for 'rhel' & 'centos'; the usage of 'rhel' has
been hardcoded.

I have only tested this patch on Scientific Linux 6.4, it likely needs additional platform testing before being merged.