Show an address field for Allocate Floating IP if user passes required policies

[Danny Cocks]

This is a feature request.

There are currently two dialogs for allocating a floating IP. One of these is available from

  Project -> Network -> Floating IPs -> Allocate IP to Project

In the code, this form is called `FloatingIpAllocate` and does not give an option to set the floating IP address. The other dialog is available from:

  Admin -> Network -> Floating IPs -> Allocate IP to Project

This form is called `AdminFloatingIpAllocate` in the code and does allow for allocating a floating IP.

By default, only a user with admin role can allocate a floating IP with a specified address. However, if the policy:

create_floatingip:floating_ip_address

is changed to allow member users to specify a floating IP, then it is possible to do this from the CLI. What would be great is to allow the same user to do this via Horizon. Currently this is not possible, as a user without admin role cannot view the "Admin" panel to go to the special Allocate IP form.

At the same time, it seems reasonable to consolidate the two forms, `FloatingIpAllocate` and `AdminFloatingIpAllocate` so that they can present similar options depending on the roles of the user.