Debugging output for docbook properties script.

* .../jenkins/files/slave_scripts/docbook-properties.sh: Have the
shell echo commands run in this script, to aid in debugging. The
console log for jobs using this didn't previously provide sufficient
feedback to be able to see what values were set in the resulting
properties file.

Change-Id: I9909009d3f85e6c3c1e5dbc5465ef38adbd75df4
Reviewed-on: https://review.openstack.org/26849
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins