Update to docbkx maven plugin 2.0.15
Update to docbkx maven plugin 2.0.15

The primary advantage of the new release is it supports specifying the
encoding for generated files, which was actually causing problems in
previous releases. The encoding is implicitly specified by setting the
project.build.sourceEncoding property (which is already in place).

Change-Id: I7cd778c6c98e23b4d92d114ef167adcfd30213b9

rackerlabs/rackspace-docs-maven-plugin,OCselected/clouddocs-maven-plugin,OCselected/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin,rackerlabs/clouddocs-maven-plugin,rackerlabs/clouddocs-maven-plugin,rackerlabs/rackspace-docs-maven-plugin,rackerlabs/rackspace-docs-maven-plugin,OCselected/clouddocs-maven-plugin,rackerlabs/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin,OCselected/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin,rackerlabs/rackspace-docs-maven-plugin