Treat default plugins like any other plugin

[kmanning]

• All plugins have an init method to enable them - all, with the exception of "default" plugins
• Default plugins are hardcoded in specific Stages. Eg: https://github.com/manheim/terraform-pipeline/blob/master/src/TerraformEnvironmentStage.groovy#L7
• Not only does hard-coding the default plugins break the init convention, it makes testing a little awkward (tests need to be written in a way that assume that default plugins are present).

Let's try to reapproach default plugins in a way that makes them consistent with every other plugin.

• Let's an an init method to default plugins, so that they conform to the convention
• Rather than hardcoding the plugins in Stages, let's call init on the default plugins in Jenkinsfile.init.
• This may break backwards compatibility. The order of plugins has an effect on the pipeline - anyone project that init's plugins before Jenkinsfile.init will now have a different order. <-- Note this incompatibility when releasing this feature.