add support for target url or target -u with dashboard url

[neo-liang-sap]

What this PR does / why we need it:
Enable gardenctl target dashboardUrl dashboard_url or gardenctl target -u dashboard_url for directly targeting garden and shoot

This requires your (virtual) garden config to have dashboardUrl field set.
E.g.

gardenClusters:
  - kubeConfig: /path/to/kubeconfig.yaml
    name: foo
    dashboardUrl: https://dashboard.foo.example.com

Which issue(s) this PR fixes:
Fixes #188

Special notes for your reviewer:
the url could be in following two formats:

• https:///namespace/garden-project/shoots/my-shoot/
• https:///namespace/garden-project/shoots/my-shoot

Release note:

Enable `gardenctl target dashboardUrl <dashboard_url>` or `gardenctl target -u <dashboard_url>` for directly targeting garden and shoot. Make sure to have `dashboardUrl` field set for each of your (virtual) garden configurations

[neo-liang-sap]

tested in following scenraio , sorry for my bad drawing skills

[neo-liang-sap]

Hi @petersutter ,
Thanks, i've fixed all issues according to your comments.
-Neo

[petersutter]

one more minor change, otherwise lgtm
thnx!

[petersutter]

@neo-liang-sap I have also updated your description and release note. PTAL

[neo-liang-sap]

thanks @petersutter for fixing the description!

[DockToFuture]

/lgtm