[JENKINS-60482] Persist the alternate ec2 endpoint

[MRamonLeon]

See https://issues.jenkins-ci.org/browse/JENKINS-60482

Simplified version of #439

Reasoning:

The field Alternate EC2 endpoint has only one purpose, to fill up the Region field. If you're in an environment where the default endpoint is not reachable, you can specify here the full url to use instead. Like: https://ec2.us-west-1.amazonaws.com. Once the field is populated, the connection and all other operations are performed using this region, so they should go fine.

It's true that every time you enter the configuration page, the Alternate EC2 endpoint is empty, so the Region field never gets populated until you set the endpoint properly. So it causes a lot of frustration and misunderstanding, IMO.

This PR tries to resolve the problem by persisting this field, so every time you reach the configuration page, the field is established, so the region field is populated correctly. This PR is simpler than #439 where we use the Alternate EC2 endpoint as the full new AWS endpoint (IIUC), which I think it's not necessary.

I've recorded a video to show the behavior: https://photos.app.goo.gl/aADg5roUcKbMqLKj8

CC: @jkfernan @GaToRAiD @troymohl

[res0nance]

Looks ok, perhaps you should add some form of test for its persistence, perhaps just extend one of the Casc test?

[MRamonLeon]

I've added a new test @res0nance , fine to you?

[res0nance]

I've added a new test @res0nance , fine to you?

Looks good