This changset allows the import of YAML formatted resources. The CLI
user can indicate which format to use with the `-f, --format` option.
The CLI help text has been amended to reflect the new feature.
The AWX CLI `export` subcommand offers the option of formatting the output
as YAML or JSON, so it makes sense that the `import` subcommand reflects
this.
A simple test is also provided. In order to ease the task of testing
commands that import resources by reading the stdin, the CLI has been
extended to allow specifying an alternative file descriptor for stdin,
similarly to stdout and stderr.
