MB-10682: Make the script relocatable

Previously we hardcoded the absolute path of the installed binary
into the wrapper script. This patch determines the path at
runtime.

Limitation:
  The directory layout have to be:

  bin/tools/program
  lib/python/python-file

Change-Id: Ia2139ec59c7cb92bdf28b2caabc504075f03f282
Reviewed-on: http://review.couchbase.org/35543
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>