Log a warning if dnsmasq version is below the minimum required

It has been noted that older versions of dnsmasq may have unexpected
behavior, so this patch introduces a check on dnsmasq version; if
the minimum version is not met, a warning is logged. This is less
invasive than raising an Exception and abort the dhcp agent, even
though this also seems like a perfectly acceptable behavior.

Fixes bug 1170793

Change-Id: Idd91c3753fb05f223ed0e3c53ad692ee65ea4905
Merge "Log a warning if dnsmasq version is below the minimum required"