Move plugin configuration to a separate file

The plugin configuration in gerrit.config is moved to events-log.config
to avoid restarting Gerrit to use new config parameter values.

This behaviour is caused by PluginConfig class, it holds a copy of
gerrit configuration file on startup, so reloading plugin will not apply
new config parameters.

Bug: Issue 9458
Change-Id: Ia823c2b32dbc00c853eb38c6dc0cd10184ca3deb