Use exportfs cli for NFS exports

Right now, NFS exports are being handled with `/etc/exports` file. However, when multiple machines are being started and thus several process writes to this file, collisions occur. I guess #1585 is related to this.

Instead of using `/etc/exports`, I've modified vagrant to utilize [exportfs](http://linux.die.net/man/8/exportfs) command line tool to dynamically export and unexport directores.

This probably won't work for OS X (at leaset I have no `exportfs` binary on my Mountain Lion machine), but this fix right now allows me to start 10 VMs in parallel with no NFS problems.

Implementation is not very clean and will probably break other hosts, but with some guide I think I can resolve all possible issues.