Add '--install-plugin' option to init step

This allows automatically install given plugin without asking. Can be
also used together with '--batch' to automatically install/update Gerrit
without interactions.

Change-Id: I8e0cf8f2e39c813e54daff6b587cb8cd2485bae8
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>

Documentation/pgm-init.txt

diff --git a/Documentation/pgm-init.txt b/Documentation/pgm-init.txt
index b9f5268..3d6cb73 100644
--- a/Documentation/pgm-init.txt
+++ b/Documentation/pgm-init.txt
@@ -13,6 +13,7 @@
 	[\--batch]
 	[\--no-auto-start]
 	[\--list-plugins]
+	[\--install-plugin=<PLUGIN_NAME>]
 
 DESCRIPTION
 -----------
@@ -49,6 +50,9 @@
 \--list-plugins::
 	Print names of plugins that can be installed during init process.
 
+\--install-plugin:
+	Automatically install plugin with given name without asking.
+
 CONTEXT
 -------
 This command can only be run on a server which has direct