Add a `disable_defaults` option to the config parser

This option is useful to force the parsing of a single configuration file,
for instance to be able to query a configuration value set in that file.
Add some features to fedmsg-config

 --query: retrieve the value of a given configuration key
 --disable-defaults: only print overridden configuration values
 --config-filename: only print the configuration for the given file
Merge pull request #222 from olasd/feature/query-config

Add some features to fedmsg-config