Add defer_permissions option

[JosephLeedy]

While trying to use the plug-in under Mac OS X 10.10.2, I kept getting "Permission denied" errors when creating or modifying files in the mounted directory. I fixed this issue by adding -o defer_permissions to line 18 of lib/vagrant-sshfs/builders/host.rb. I found the solution in this issue for OSXFuse.

This issue could also be fixed with the solution provided in pr #30.

Background

I am mounting a Centos 6.6 guest's Apache web root (in /srv/www) to a local folder. The web root is owned by the apache user and group, and there is an ACL configured to give the vagrant user full rwx permission.

I had to install OSXFuse and it's SSHFS plug-in to get this working because I was getting an error saying that the sshfs binary could not be found. The plug-in's README file mentions a requirement for SSHFS being installed on the guest, but not the host which was confusing at first.