Fixed useradd to create home dir totally empty

When used with -m, useradd creates a home directory, BUT it also adds a bunch of .hidden files (.bashrc, etc) by default. Using the -k /dev/null prevents that from happening. 

This is important because git clone can only clone a repository to a COMPLETELY empty dir (a few steps later in the process). If the .hidden files are present, the cloning will fail.