Create /etc/fstab file for Ubuntu stemcells

When attempting to deploy with a custom Ubuntu stemcell on Openstack, we encountered the following error due to /etc/fstab being empty. 

"mounting /var/lib/nova/instances/e562b310-d5db-4833-992a-3647a4808bb2/disk with libguestfs (could not parse /etc/fstab or empty file))"

 While looking through the stemcell_builder scripts, we saw that the following line is appended to the CentOS stemcell
```
# /etc/fstab Created by BOSH Stemcell Builder
UUID=${uuid} / ext4 defaults 1 1
```
So, we modified this script to append the same line to Ubuntu Stemcells, which allowed us to bypass our problem.