Fully-qualify the path to cat

fixes this error:
Error: Validation of Exec[compile-nginx-config] failed: 'cat
/etc/nginx/conf.d/.frag* > /etc/nginx/conf.d/generated.conf' is not
qualified and no path was specified. Please qualify the command or
specify a path. at /etc/puppet/modules/nginx/manifests/service.pp:26
Merge pull request #2 from AndrewLorente/master

Fully-qualify the path to cat