Add support for remote API

This change adds remote API support. You can get the status of Gerrit connection using the below url:

$JENKINS_URL/plugin/gerrit-trigger/api/(xml|json)

Sample:

```xml
<pluginImpl>
  <server>
    <connected>true</connected>
    <frontEndUrl>http://localhost:8080/</frontEndUrl>
    <hostName>localhost</hostName>
    <name>gerrit1</name>
    <noConnectionOnStartup>false</noConnectionOnStartup>
    <sshPort>29418</sshPort>
    <timeoutWakeup>false</timeoutWakeup>
    <userName>foobar</userName>
    <httpUserName>foobar</httpUserName>
  </server>
</pluginImpl>
```

Also some small fixes:

* No keep plugin instance in PluginImpl
* Use WriteOnArrayList as server list (improve UI performance)

I think this change is OK, I just wanted to try to avoid this type of change if it is possible, but there is no big harm here I hope.
I was supposed to have stopped a method that returns specific List implementations when it showed up to be gin with, but I was too lazy at the time of merging it.