user story #2191: give jenkins plugins providers option

- This adds a providers option to jenkins plugins so that it
  can now download a plugin from url and install it.
- It behaves just as before with default options, and option
  to specify an alternative jenkins plugin root and plugin folder.
- You can now specify a different jenkins user / group
- you can specify the download md5 when using the provider download.
- You can also specify ensure property absent to remove plugins.

Change-Id: Ieb3e30d3211fd87e688cf054c2b156a29754b25d