Add NOPASSWD to sudoers example for running virsh

Without the NOPASSWD tag, smart-proxy fails
to run virsh and the following error is logged:

ERROR -- : DHCP virsh provider error: unable
to retrive virsh info: DNS virsh provider error:
virsh call failed (pid 569 exit 1) - sudo: no tty
present and no askpass program specified
Merge pull request #165 from jbrouault/patch-1

Add NOPASSWD to sudoers example for running virsh