Restrict deep for yaml hierarchy

[den-gts]

At this moment plugin did handle all levels in yaml tree.
For example next yaml

root:
    a:
        nightly_check: true
   b:
        nightly_check: false
   c:
        nightly_check: true

will return for Axis name (Top key of yaml) = root

[a={nightly_check=true}, b={nightly_check=false}, c={nightly_check=true}]

and configuration's names will be:

a={nightly_check=true}
b={nightly_check=false}
c={nightly_check=true}

Will be great if plugin will return:

[a,b,c]

[sue445]

@den-gts Please add test case to followings.

• https://github.com/jenkinsci/yaml-axis-plugin/blob/0.2.0/src/test/groovy/org/jenkinsci/plugins/yamlaxis/YamlTextLoaderSpock.groovy#L6-L34
• https://github.com/jenkinsci/yaml-axis-plugin/blob/0.2.0/src/test/groovy/org/jenkinsci/plugins/yamlaxis/YamlFileLoaderSpec.groovy#L10-L25

BTY I revived the CI at #16 . Please rebase from latest master

[den-gts]

I'm stucked at:

General error during class generation: java.lang.NoClassDefFoundError: Unable to load class hudson.matrix.MatrixProject due to missing dependency hudson/tasks/test/AggregatedTestResultAction

That error reproduced at my local environment and your CI while TestCases gradle task. I believe I should update version of some component.