Configuration changes - Fallback to gardenctl-v2 config and property name changes

[petersutter]

What this PR does / why we need it:

1. ⚠️ the configuration properties have changed to align with gardenctl-v2. The following properties were marked deprecated. The local config file should be adapted accordingly:

• gardenClusters was renamed to gardens
• clusterIdentity was renamed to identity

2. If no configuration file is found (~/.garden/gardenlogin.yaml), it falls back to the gardenctl-v2 configuration file (~/.garden/gardenctl-v2.yaml) which shares the same configuration properties.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

⚠️ the configuration properties have changed to align with `gardenctl-v2`. Please rename your configuration properties accordingly as they may not be supported with future `gardenlogin` releases:
- `gardenClusters` was renamed to `gardens`
- `clusterIdentity` was renamed to `identity`

If no configuration file is found, it falls back to the `gardenctl-v2` configuration file (`~/.garden/gardenctl-v2.yaml`) which shares the same configuration properties

[holgerkoser]

/lgtm