Merge and rename generatedocbook/generatepot

The tool scripts used common functions, merge them to a single file
with different entry points.

Cleanup the merged file so that it passes the pep8 test.

Install the commands now as openstack-generate-docbook and
openstack-generate-pot. Leave a compatibility wrapper which should be
removed in version 0.8.

Change-Id: I7e4d651c4c62df58499e64e38528cb2cc4e9175b