Fix PATH issue in sudoers file for CentOS-5.5-x86_64-netboot

Added sed command to postinstall.sh to include PATH in the env_keep
block in /etc/sudoers. Makes it so vagrant is able to detect chef and
puppet on the VM.
Merge pull request #536 from erikthered/centos-5.5-path

Fix PATH issue in sudoers file for CentOS-5.5-x86_64-netboot