Add option to transform JSON response

[lukas-vlcek]

Most commands accept --transform=nested option. It transforms JSON response to nested data (this means the JSON does not contain data as a JSON keys).

Make tests pass on clusters with more than one node.

Closes #24

[lukas-vlcek]

@richm PTAL

[lukas-vlcek]

@richm I did some modifications. The option is called --transform=nested now and the code is more general & prepared to support more transformation types in the future.

[richm]

Add a test like this e.g. to tests/commands/test_cluster_state.py:

    def test_bogus_transform_value(self):
        cmd = self.appendSecurityCommands(['watches', 'cluster_state', '--transform=bogus'])
        output, errout = popen(cmd, stdout=PIPE, stderr=PIPE).communicate()
        self.assertRegexpMatches(errout, '(?ms)ERROR:.*RuntimeError: Unsupported transform type')

I think you only need to add a test like this to one of the commands that supports the --transform argument.

[lukas-vlcek]

Done (in tests/commands/test_cluster_health.py).