[tctl] Don't explicitly set value for config path and preserve backwards compatibility

[webvictim]

Fixes #5725

Historically, this is tctl's precedence order for OSS (most important at the top):

• CLI flags
• Values from config file provided on command line via -c/--config
• Values from Teleport's default config file path (/etc/teleport.yaml) if present
  • if this file is not present, ignore it and continue
• Defaults

This precedence order was changed (and was broken) in #5483 (changes in bold):

• CLI flags
• Values from config file provided on command line via -c/--config
• Values from config file provided via TELEPORT_CONFIG_FILE environment variable
• Values from Teleport's default config file path (/etc/teleport.yaml)
  • if this file is not present, throw an error and exit
• Defaults

This PR changes it to the following (which was the original intention; changes in bold):

• CLI flags
• Values from config file provided on command line via -c/--config
• Values from config file provided via TELEPORT_CONFIG_FILE environment variable
• Values from Teleport's default config file path (/etc/teleport.yaml)
  • if this file is not present, ignore it and continue
• Defaults

Also moves a couple of copies of the same fileExists function into utils for reuse.