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Enhancements to installation script: ``--version`` and ``--uninstall``
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Use case
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User might wish to control which rally version is installed or even purge
rally from the machine completely.


Problem description
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    #. Installation script doesn't allow to choose version.
    #. No un-install support.



Possible solution
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    #. Add ``--version`` option to installation script.
    #. Add ``--uninstall`` option to installation script or create an
       un-installation script