Fix DictOpt to split only the first colon

I've encountered a ValueError when adapting DictOpt to
neutron's network_vlan_ranges.
https://github.com/openstack/neutron/blob/master/etc/neutron/plugins/openvswitch/ovs_neutron_plugin.ini#L23

This patch makes dictopt to split only the first colon for every value

Fixes bug #1223667

Change-Id: Iee4011f2a45be933dad393c72df099d6a8116849