Removed trailing whitespace from help output

Some of the output help from *-manage CLIs (eg. trove-manage) have
whitespace at the end of the line.
This causes the gate-openstack-manuals-tox-checkniceness test to fail
on the generated .xml files.

Trailing white-space is now stripped off by the tool before writing the
.xml file.

Change-Id: I94aa3a02f7bbe2e88beb510551a2aff76ed24f8a
Closes-Bug: #1342988