Cleanup to clear five compiler warnings.

Fixes the following classes of warnings during the compile phase:

1. Delayed init likely to yield an uninitialized value:

    ```
    [warn] src/main/scala/mesosphere/marathon/Main.scala:24: Selecting value zooKeeperTimeout from trait ZookeeperConf, which extends scala.DelayedInit, is likely to yield an uninitialized value
    [warn]       conf.zooKeeperTimeout() < Integer.MAX_VALUE,
    [warn]            ^
    ```

1. Nullary methods with side-effects are discouraged:

    ```
    [warn] src/main/scala/mesosphere/marathon/api/v2/EventSubscriptionsResource.scala:49: side-effecting nullary methods are discouraged: suggest defining as `def validateSubscriptionService()` instead
    [warn]   private def validateSubscriptionService = {
    [warn]               ^
    ```