Don't write metadata file in `bento build` dry run mode.
Don't write metadata file in `bento build` dry run mode.

Fixes #368

legal90/bento,techmaniack/bento,bgandon/bento,juliandunn/bento,legal90/bento,lwieske/bento,juliandunn/bento,chef/bento,davidalger/bento,norcams/bento,leejones/bento,devopxteam/bento,leohmoraes/bento,ConSol/bento,devopxteam/bento,lwieske/bento,dthtvwls/bento,vincentaubert/bento,patcon/bento,thinktainer/bento,mmckinst/bento,grdryn/bento,risdenk/bento,loadsys/packer-builds,sillvan/bento,chef/bento,tas50/bento,Sauraus/bento,0x90/bento,RallySoftware-cookbooks/bento,erikj/bento,loadsys/packer-builds,risdenk/bento,dmlb2000/bento,patcon/bento,techmaniack/bento,juliandunn/bento,techmaniack/bento,jhoblitt/bento,Oefenweb/bento,patrickmcclory/bento,Sauraus/bento,William-Yeh/bento,leohmoraes/bento,norcams/bento,runsystem-hiennt2/bento,b1-systems/bento,runsystem-hiennt2/bento,leejones/bento,norcams/bento,Oefenweb/bento,bgandon/bento,vincentaubert/bento,patrickmcclory/bento,ConSol/bento,dmlb2000/bento,sporting-news/bento,v1k0d3n/bento,tas50/bento,devopxteam/bento,sporting-news/bento,lwieske/bento,mmckinst/bento,legal90/bento,davidalger/bento,William-Yeh/bento,orpiske/bento,erikj/bento,stumped2/packer,grdryn/bento,stumped2/packer,jhoblitt/bento,vincentaubert/bento,tas50/bento,thinktainer/bento,v1k0d3n/bento,chef/bento,0x90/bento,dthtvwls/bento,sillvan/bento,loadsys/packer-builds,orpiske/bento,RallySoftware-cookbooks/bento,b1-systems/bento,devopxteam/bento,ConSol/bento