Add useJetty configuration option to change default server.

We are using the standalone plugin as a dependency for another build related
plugin which packages the grails application as an RPM.  My plugin calls
buildStandalone() within it's own GANT task.  Since the standalone plugin
relies completely on command line arguments to change the server, it makes using
Jetty instead of Tomcat very difficult in this manner.

By adding the configuration option, we gain the necessary flexibility without
losing any current functionality.  A user can now use the following config
option to make Jetty the default:

grails.plugins.standalone.useJetty = true

A complementary flag is being added as well, which will allow you to override
the Jetty default in the same manner you would override the Tomcat default:

grails build-standalone --tomcat