Prepare release 3.0.0

- document key matching behaviour
- bump versions in README.md
- treat matched key like an existing key

Author: JonasProgrammer

README.md

@@ -15,8 +15,8 @@ You can find sources and pre-compiled binaries [here](https://github.com/JonasPr
 
 ```bash
 # Download the binary (this example downloads the binary for linux amd64)
-$ wget https://github.com/JonasProgrammer/docker-machine-driver-hetzner/releases/download/2.1.0/docker-machine-driver-hetzner_2.1.0_linux_amd64.tar.gz
-$ tar -xvf docker-machine-driver-hetzner_2.1.0_linux_amd64.tar.gz
+$ wget https://github.com/JonasProgrammer/docker-machine-driver-hetzner/releases/download/3.0.0/docker-machine-driver-hetzner_3.0.0_linux_amd64.tar.gz
+$ tar -xvf docker-machine-driver-hetzner_3.0.0_linux_amd64.tar.gz
 
 # Make it executable and copy the binary in a directory accessible with your $PATH
 $ chmod +x docker-machine-driver-hetzner
@@ -95,7 +95,7 @@ $ docker-machine create \
 - `--hetzner-image-id`: The id of the Hetzner cloud image (or snapshot) to use, see [Images API](https://docs.hetzner.cloud/#resources-images-get) for how to get a list (mutually excludes `--hetzner-image`).
 - `--hetzner-server-type`: The type of the Hetzner Cloud server, see [Server Types API](https://docs.hetzner.cloud/#resources-server-types-get) for how to get a list (defaults to `cx11`).
 - `--hetzner-server-location`: The location to create the server in, see [Locations API](https://docs.hetzner.cloud/#resources-locations-get) for how to get a list.
-- `--hetzner-existing-key-path`: Use an existing (local) SSH key instead of generating a new keypair.
+- `--hetzner-existing-key-path`: Use an existing (local) SSH key instead of generating a new keypair. If a remote key with a matching fingerprint exists, it will be used as if specified using `--hetzner-existing-key-id`, rather than uploading a new key.
 - `--hetzner-existing-key-id`: **requires `--hetzner-existing-key-path`**. Use an existing (remote) SSH key instead of uploading the imported key pair,
   see [SSH Keys API](https://docs.hetzner.cloud/#resources-ssh-keys-get) for how to get a list
 - `--hetzner-user-data`: Cloud-init based User data

driver.go

@@ -253,15 +253,18 @@ func (d *Driver) Create() error {
 			key, _, err = d.getClient().SSHKey.Create(context.Background(), keyopts)
 			if err != nil {
 				return errors.Wrap(err, "could not create ssh key")
+			} else if key == nil {
+				return errors.Errorf("key upload did not return an error, but key was nil")
 			}
+
+			d.danglingKey = true
+			defer d.destroyDanglingKey()
 		} else {
+			d.IsExistingKey = true
 			log.Debugf("SSH key found in Hetzner. ID: %d", key.ID)
 		}
 
 		d.KeyID = key.ID
-		d.danglingKey = true
-
-		defer d.destroyDanglingKey()
 	}
 
 	log.Infof("Creating Hetzner server...")