Use keyring only if it exists

[davidferlay]

Observed

• It make sense for user to execute plasmactl update anywhere on system
  • Including in a directory where no keyring exists, like user home or system root

Expected

• To avoid creating keyring files in random place of system, it's probably best not for update command not to have abiltiy to create new keyrings
• That way, command will use keyring if it already exists (in most case, project directory) and not polute the system otherwise
• So in case keyring not found in ./{binary-name}/keyring.yaml.age of current directory, then prompt user for credentials, to be used right away and not be stored anywhere