Fix plot plugin support

Based on the source code of the jenkins plot plugin
https://github.com/jenkinsci/plot-plugin/ the related publisher
has been updated to generate a working XML block and match the
behaviour for data persistency in CSV file with random generated
name.
Minor code adjustement to match coding covention (Hyphen and
capital letter usage)
Plot group now correctly handled.
All three type of plot with different option are covered in tests.

Change-Id: I6bd22ea19f397cf2123b924ff5517682a53177ed
Closes-Bug: #1223017
Merge "Fix plot plugin support"