cork: add --json-key to create and download

[tormath1]

In this PR, we add the --json-key flag to create and download subcommands from cork. It allows a user to pass a GCS credential file path to cork in order to download from a private GCS bucket.

update command use the flags from the creation flags set, so no need to update this part:

updateCmd.Flags().AddFlagSet(creationFlags)

Testing done

$ ./bin/cork create --verbose --sdk-url-path "/flatcar-jenkins/sdk"
2021-10-04T16:11:35Z cli: Started logging at level INFO
2021-10-04T16:11:35Z cork: Detecting SDK version
2021-10-04T16:11:36Z cork: Found SDK version 3005.0.0 from remote repo
2021-10-04T16:11:36Z cork: Downloading SDK version 3005.0.0
2021-10-04T16:11:36Z sdk: Downloading https://storage.googleapis.com/flatcar-jenkins/sdk/amd64/3005.0.0/flatcar-sdk-amd64-3005.0.0.tar.bz2 to /home/mathieu/go/src/github.com/kinvolk/mantle/.cache/sdks/flatcar-sdk-amd64-3005.0.0.tar.bz2
2021-10-04T16:11:41Z cork: Download failed: 403 Forbidden: https://storage.googleapis.com/flatcar-jenkins/sdk/amd64/3005.0.0/flatcar-sdk-amd64-3005.0.0.tar.bz2
$ ./bin/cork create --json-key=$HOME/.gce/release.json --verbose
2021-10-04T16:05:55Z cli: Started logging at level INFO
2021-10-04T16:05:55Z cork: Detecting SDK version
2021-10-04T16:05:56Z cork: Found SDK version 3005.0.0 from remote repo
2021-10-04T16:05:56Z cork: Downloading SDK version 3005.0.0
2021-10-04T16:05:56Z sdk: Downloading https://storage.googleapis.com/flatcar-jenkins/sdk/amd64/3005.0.0/flatcar-sdk-amd64-3005.0.0.tar.bz2 to /home/mathieu/go/src/github.com/kinvolk/mantle/.cache/sdks/flatcar-sdk-amd64-3005.0.0.tar.bz2
2021-10-04T16:05:56Z sdk: Resuming from byte 21495808
^Catcar-sdk-amd64-3005.0.0.tar.bz2: [                          ] 36.1 MB/1.34 GB