Add jenkins::plugin defined resource type

This commit adds jenkins::plugin which can be used
to install jenkins plugins. I used it to install
gearman just today:

  jenkins::plugin { 'gearman-plugin':
    version => '0.0.3',
  }

Note: this code is borrowed from:

  https://github.com/jenkinsci/puppet-jenkins/blob/master/manifests/plugin.pp

which is apache licensed.

Change-Id: Ifd3f6452839db990357f14665f14aa839ec2d0c5
Don't automatically restart jenkins on plugin changes

This way we can manually gracefully restart jenkins.  Since this
won't immediately install plugins, change the download test to
check whether an hpi or jpi file exists.

Change-Id: I41667684ad8a57f5ae554f866dcc38d5ecca6ba0
Add current jenkins plugins

Change-Id: Id02e07a78a02b65b1f0776fc5321fa75993324c0
Merge "Add jenkins::plugin defined resource type"
Merge "Don't automatically restart jenkins on plugin changes"
Merge "Add current jenkins plugins"