[FLINK-4622] CLI help message should include 'savepoint' action

[skidder]

The Flink CLI help message should include the 'savepoint' action in the list of available actions. It currently looks like:

bash-4.3# flink foo
"foo" is not a valid action.

Valid actions are "run", "list", "info", "stop", or "cancel".

Specify the version option (-v or --version) to print Flink version.

Specify the help option (-h or --help) to get help on the command.

This pull-request adds the savepoint action to the list of valid actions.

[StephanEwen]

Thanks for the correction.

+1 to merge this!

[zentol]

merging