Refactor outlier detection configuration fields

*Title*: Refactor outlier detection config message fields

*Description*:
As discussed in https://github.com/envoyproxy/envoy/pull/8130#discussion_r320832679, the outlier detection configuration fields are a bit messy and could use a cleanup. The main work here is to de-dup all of the options into a common message around enabled, local origin split, etc. and then hopefully just have a single field per actual ejection type. This would avoid an explosion of almost identical fields for each different ejection type.