dhcp should be set to enabled when an ipv6 address mode is selected

[Ying Zuo]

Steps to reproduce:
1. Enable ipv6 on the local_settings.py
2. Create a network
3. Create an ipv6 subnet. On the subnet details tab, uncheck the Enable DHCP checkbox and select something other than "No options specified" in the IPV6 address configuration mode dropdown.
4. An error is shown indicating the enable_dhcp needs to be set to True for the subnet to be created.

According to the section Address modes for ports on http://docs.openstack.org/newton/networking-guide/config-ipv6.html#address-modes-for-ports, DHCP should be forced to be enabled in this case.

[Akihiro Motoki (amotoki)]

I think there are several options.

(a) Determine the actual enable_dhcp value based on the IPv6 mode selection. In this case, it is better that "Enable DHCP" field is shown only when IPv4 is chosen and is hidden if IPv6.
(b) Show a proper error message that "Enable DHCP" needs to be checked when an IPv6 mode other than "No options specified" is selected.
(c) Hide IPv6 modes menu when "Enable DHCP" is disabled.

My vote is (a) or (b). (c) might be confusing.

[Ying Zuo (yingzuo)]

Thanks for listing the options. I think option b is the best. I will try to work on this soon.