Add a function to discover the input.xml file

Currently, the path to input.xml is hardcoded and, when Aquilon is
installed from RPM, resolves to /usr/etc/input.xml, which is wrong.

To keep the old installation procedure working, we need to discover this
file at run time: if the old path exists, it is used, otherwise we fall
back to the system-wide configurations.

We need it only in the client aq executable.  Everywhere else seems to
be getting its input.xml file from external parameters.
Use get_parser in the aquilon client script
Add unit tests for get_parser method