Update host.rb

mapall= and maproot= are mutually exclusive options. Currently, when trying to use maproot=, mapall= is getting automatically added, which prevents us from using the NFS mount. This change turns off the adding of mapall= if maproot= has been specified as an option.
Merge pull request #2448 from rimckenn/patch-1

hosts/bsd: don't set both maproot and mapall