add default cookbook attribute for ['omnibus']['cache-dirs']
add default cookbook attribute for ['omnibus']['cache-dirs']

Fix to stop your friendly neighborhood "undefined method `each' for
nil:NilClass" exception when the omnibus::default recipe is executed.
Existing (*nix) Omnibus projects do not have this value set in their
Vagrantfile's chef.json.

rhass/omnibus,intfrr/omnibus,hugosato/omnibus,chef/omnibus,adamleff/omnibus,intfrr/omnibus,chef/omnibus,chef/omnibus,higanworks/omnibus,rapid7/omnibus,jamesn/omnibus,ashhher3/omnibus,rhass/omnibus,pskrz/omnibus,lvfeng1130/omnibus,higanworks/omnibus,szechyjs/omnibus,sh9189/omnibus,Altiscale/omnibus,adamleff/omnibus,sh9189/omnibus,higanworks/omnibus,DataDog/omnibus-ruby,Altiscale/omnibus,lvfeng1130/omnibus,sh9189/omnibus,hugosato/omnibus,DataDog/omnibus-ruby,3ofcoins/chef-omnibus,Scalr/omnibus,hugosato/omnibus,3ofcoins/chef-omnibus,rapid7/omnibus,Altiscale/omnibus,pskrz/omnibus,3ofcoins/chef-omnibus,mattray/omnibus,strangelittlemonkey/omnibus,jamesn/omnibus,servebox/omnibus,pskrz/omnibus,Scalr/omnibus,jamesn/omnibus,Scalr/omnibus,DataDog/omnibus-ruby,mattray/omnibus,ashhher3/omnibus,mattray/omnibus,intfrr/omnibus,adamleff/omnibus,servebox/omnibus,lvfeng1130/omnibus,ashhher3/omnibus,szechyjs/omnibus,strangelittlemonkey/omnibus,rapid7/omnibus,rhass/omnibus,servebox/omnibus,szechyjs/omnibus,strangelittlemonkey/omnibus