Add get_plugins_info and get_plugin_info methods

This change adds methods to retrieve jenkins plugin information.

get_plugins_info - will return a list containing all installed plugins.

get_plugin_info - will query (by name) for a specific plugin and return
  info on that plugin.

Change-Id: Ia6cd90571ff260aec9869f48df06486b7fac181f
Merge "Add get_plugins_info and get_plugin_info methods"