Instance networking: attach ephemeral IP button

[david-crespo]

We're adding ephemeral IP detach in #2285. An attach ephemeral IP button would make sense next to the floating IP button. There is a max of 1 ephemeral IP per instance (nexus source), so we should disable the button whenever there is already an ephemeral IP.

[charliepark]

Factor in #2277, which notes that

External IP option is a no-op if user has chosen "none" for network interface.