matrix project: add support for dynamic axis

This axis requires the dynamic axis plugin:
https://wiki.jenkins-ci.org/display/JENKINS/DynamicAxis+Plugin

All the dynamic axis plugin settings are in the core project:

<matrix-project>
  <axes>
    <ca.silvermaplesolutions.jenkins.plugins.daxis.DynamicAxis>
      <name>config</name>
      <values>
        <string>config_list</string>
      </values>
      <varName>config_list</varName>
      <axisValues>
        <string>default</string>
      </axisValues>
    </ca.silvermaplesolutions.jenkins.plugins.daxis.DynamicAxis>
  </axes>

The plugin sets 2 values: axis name and variable name.
The axisValue element is hardcoded to 'default' in the plugin, so we do
the same in Jenkins job builder.

The matrix project supports multiple axes configuration, including dynamic
axis.

Tests added for:
 * dynamic axis (by itself)
 * multiple axes (dynamic and user-defined axes)

Tested on Jenkins LTS (1.554.2) with both the plugin installed and not
installed. The plugin is properly ignored in the latter case.

Change-Id: I36310bc512fd873fec4bd5f42937689fd178536e
Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Merge "matrix project: add support for dynamic axis"