Default cluster image for this release

The default cluster image for this release is radanalyticsio/openshift-spark:2.2-latest found here
The Apache Spark version is 2.2.

Summary of changes

Allow spark metrics to be enabled during cluster creation (default is disabled).
Metrics may be enabled in three different ways:
- via the --metrics=[true|false] flag on "oshinko create"
- by setting a "metrics" value in a cluster configmap (true or false)
- by setting the "metrics" config value when creating a cluster with the REST api (true or false)
Modify the handling of the "exposeui" flag for consistency:
- if the --exposeui flag is not set on "oshinko create", read the value from a configmap or the default (True)
- allow "exposeui" to be set in a cluster configmap
- make the "exposeui" value a string in the REST api instead of a boolean