Fix DHCP agent to work with latest dnsmasq

Fix DHCP agent to work with latest dnsmasq

The latest dnsmasq no longer accepts hostnames which begin with a number. This
affects Fedora 19 right now, and will mean Fedora 19 will not work with Neutron
DHCP. dnsmasq should work with hostnames beginning with a number (RFC 1123 says
this is valid), but until this is fixed, many users will be left unable to use
Fedora 19 with Neutron.

This patch adds a "host-" prefix to each hostname entry generated by the DHCP
agent. This fixes the issue in Neutron.

Fixes bug 1204125

Change-Id: I0e29ec033969c3fb958ed3a12b8962b73b0e3d94