(FM-1907) Fix mod_negotation system tests

Previously we were including the base apache class and then including the
'apache::mod::negotiation' class. However the apache class includes
'apache::mod::negoatiation' by default. Telling apache not to use its
default mods fixes this error.

There was also a reference to '$mod_dir' when finding the negotiation.conf
instead of just 'mod_dir'.

s/puppet code/ruby code/g fixed the issue. :)
Merge pull request #858 from justinstoller/bug/master/1907-fixup-mod_negotiation

(FM-1907) Fix mod_negotation system tests