Support local cloud provider

[grolu]

What this PR does / why we need it:
This PR has several improvements regarding unsupported cloud providers.

If no (valid) cloud provider is configured that the dashboard supports, this will no longer result in an fatal error 500. Instead this will be rendered as an information in the dashboard

Added a generic secret dialog as well as fallback implementation for all required code parts. Start implementing support for a new cloud provider is now as simple as adding the name of the new provider to the list of supported providers. This will be further enhanced with follow-up PR #1436

Added support for Gardener local infrastructure provider

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

If no (valid) cloud provider is configured that the dashboard supports, this will no longer result in an fatal error 500. Instead this will be rendered as an alert in the dashboard

Added support for `local` infrastructure provider. This allows to use the dashboard with local gardener installations. See also: [Deploying Gardener locally](https://github.com/gardener/gardener/blob/master/docs/deployment/getting_started_locally.md)

Fixed: The Dashboard did not recognize `providerTypes: ['*']` as valid `seedSelector`

Fixed: Seeds in other regions than the shoot did not show up if no `seedCandidateDeterminationStrategy` set in dashboard config

[gardener-robot]

@holgerkoser You have pull request review open invite, please check

[petersutter]

/lgtm