Parameterize grails central

* [Grails central should be configurable just like maven central](https://github.com/rundeck/rundeck/blob/76199662715a0cce0bde618f19df4d4d8f4a7f07/rundeckapp/grails-app/conf/BuildConfig.groovy#L59) (akin to https://github.com/rundeck/rundeck/pull/321).


* Although the [cleanGrails task](https://github.com/rundeck/rundeck/blob/af47bba8c69dfb35e67001b0bad8a1fc575632b9/rundeckapp/build.gradle#L202) is configured to ignore the exit value, it'd be nice to clean up the output when grails and/or maven central are unavailable.  This means that it should have the same maven (and grails) central override logic as the other tasks.

* Per the [grails docs](http://grails.org/doc/2.2.4/guide/conf.html#pluginDependencies), grails plugins should no longer be declared in [application.properties](https://github.com/rundeck/rundeck/blob/76199662715a0cce0bde618f19df4d4d8f4a7f07/rundeckapp/application.properties).  Instead, they should be declared in [BuildConfig.groovy](https://github.com/rundeck/rundeck/blob/76199662715a0cce0bde618f19df4d4d8f4a7f07/rundeckapp/grails-app/conf/BuildConfig.groovy#L75).  Additional discussion can be found [here](http://grails.1312388.n4.nabble.com/Plugins-application-properties-vs-BuildConfig-groovy-td4313370.html).