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/35496
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>