Add ability to define custom dhcp ranges

Allows the user to set smaller dhcp ranges so that openstack
may have the ability to define floating ips based on the remaining
pool.

Fixes: #220
Fixes: #218

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Merge pull request #221 from Ubuntu-Solutions-Engineering/patch-allow-customized-dhcp-ranges

Add ability to define custom dhcp ranges