New Parent POM for Jenkins Plugins

Propose a revision of the parent plugin POM is to decouple this artifact from the Jenkins Core:

• Simplifying the mechanism to build and test a plugin against different core versions.
• Decoupling build-related aspects, such as static analysis tools, JRE signatures, etc. from the baseline core versions, as they are totally independent concerns, reducing the need to include otherwise common configuration in each plugin POM just because we want to support and older baseline.

One use case that would greatly benefit from this change is for example, jenkins#1530, where it is necessary to propose API changes in core and matching plugin usages. The current plugins/pom.xml does not work if you mvn deploy a SNAPSHOT revision. Plugin Compatibility Testing can also benefit from this change.