We use "gerrit" as a plugin name when a plugin extension point is
implemented by Gerrit core. The string for this plugin name was
hard-coded in many places. It is better to have a constant for this.
Change-Id: I032812d4f6e8d63197cd36e3b0399f60094725cf
Signed-off-by: Edwin Kempin <ekempin@google.com>
5b337c20e8