Corrected static token handling.

[russjones]

Purpose

Change the behavior of how static tokens are handled to match what users expect.

1. Starting Teleport with no configuration file will set static tokens to an empty list on the backend.
2. Starting Teleport with a configuration file but nothing defined for the tokens field will set static tokens to an empty list on the backend.
3. Starting Teleport with a configuration file with tokens: [] will set static tokens to an empty list on the backend.
4. Starting Teleport with a configuration file with tokens: ["node,auth,proxy:foo"] will set static tokens to an node,auth,proxy:foo backend.

Implementation

• When Teleport starts it creates default configuration with static tokens set to an empty list.
• If the file configuration contains values for the tokens field, then service.Config is updated with the tokens in file configuration.
• Upon every start, static tokens are pushed to the backend overwriting whatever existed in the backend.
• When listing tokens, only list tokens if they exist otherwise don't append anything to the list of tokens.

Related Issues

Fixes #1348