Improve CapiProvider edit behaviour

[mantis-toboggan-md]

We need to determine which of the CapiProvider fields are editable: currently, the UI allows users to change everything except name and namespace with the built-in provider options. The custom capiprovider edit options are more limited -- there is a related issue where we are not showing those input fields, but we have the same problem re: not knowing which should be editable.

An additional, related concern is if and when users should be allowed to delete these capiproviders. Currently, deletion is unrestricted.

create options:

edit options:

[mantis-toboggan-md]

I filed a separate issue to look into the hidden fields when editing custom providers; it looks like a UI bug to me #105

[Danil-Grigorev]

From the high level overview, editable should be:

1. Description
2. Version + version can be set to empty to use latest
3. Anything in the secrets section
4. Anything in the Features section
5. Anything in the Variables section

No need to provide defaults for these features on creation/edit, it is best to rely on turtles defaults per provider.