Attach interface: require ip specificied if no allocation_pools set

[notandy]

Elektra shows a error messaging for attach-interfaces if the subnet has no attribute 'allocation_pools' set. In such case neutron cannot auto-allocate an ip address and the user should be forced to set an address manually:

[andypf]

@notandy Do you have a suggestion how we can solve it? I would suggest adopting the description of the field:

"Choose a previously reserved IP or type a new one. Leave empty to let the DHCP select one for you (this requires the backend to set allocation_pools)."