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' as of facter >= 1.7.2

* gentoo was improperly part of the amazon package set; this patch removes
support for gentoo but it was broken anyways

modifications to nginx::package::redhat were made as well

* it no longer tries to setup the nginx.org yumrepo for fedora as no packages
for fedora are currently provided

* amazon release numbers are inconsistent with EL.  Unknown
$::lsbmajdistrelease values are now mapped to 6 so it's no longer nessicary to
test for $::lsbmajdistrelease being undefined.  This logic will need to be
reworked after RHEL7.x is released.

* the url to the nginx repo was including $::operatingsystem in it but
nginx.org only has package dirs for 'rhel' & 'centos' which are presently
identical; the usage of the 'rhel' dir has been hardcoded.  This fixes broken
yum repo setup for all $::osfamily == 'redhat' platforms other than redhat and
centos.
Merge pull request #99 from jhoblitt/select_package_by_osfamily

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