Use qualified path to invoke scripts

* Add a config var, 'bin_dir', which specifies the path to scap's bin dir.
* Add cli.Application#get_script_path, which resolves a script name to a
  qualified file path.
* Use the above to make script invocations use qualified paths.

Change-Id: I083d6e58ecd68a997dd78faabe60a3eaf8dfaa3c