Define %rhel macro

We currently don't define the rhel macro in our mock environments, but we
reference it in specs. el6 provides %rhel, but el5 doesn't. This commit updates
the rpmbuilder module to define the macro in the mock configs we lay down
always.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #23 from MosesMendoza/maint/master/add_rhel_macro

Define %rhel macro