CLOUDSTACK-7974: remove old hostname entry for a VM when adding a VM

When adding a VM, it adds an entry to /etc/hosts file on the VR but does not
clear up any older entries for the VM with a same name. The fix uncomments the
command that removes any old entries in the VM.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>