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/program
  lib/python/python-file

Change-Id: I23af76bbe3d5c3036474c4a40c03b2ad7cfcfe1e
Reviewed-on: http://review.couchbase.org/35498
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>