Refactor and fix status() in repository_tool.py.

Update and refactor status() following the changes to how metadata is written.
Minor comment change to conf.py.
Example output:
'root' role contains 1 / 1 signatures.
'targets' role contains 1 / 1 signatures.
'release' role contains 1 / 1 signatures.
'timestamp' role contains 1 / 1 signatures.

# Verify invalid number of public and private keys.
'timestamp' role contains 0 / 1 signing keys.

# Determine the delegated roles with invalid metadata.
Delegated roles with insufficient keys:
['targets/unclaimed/1', 'targets/unclaimed/0', 'targets/unclaimed/2', 'targets/unclaimed/5', 'targets/unclaimed/4', 'targets/unclaimed/7', 'targets/unclaimed/6', 'targets/unclaimed/9', 'targets/unclaimed/f', 'targets/unclaimed/3', 'targets/unclaimed/a', 'targets/unclaimed/c', 'targets/unclaimed/b', 'targets/unclaimed/e', 'targets/unclaimed/d', 'targets/unclaimed/8']