Simplify opportunistic container parameters and metrics

This JIRA removes some of the parameters that are related to opportunistic containers (e.g., min/max memory/cpu). Instead, we will be using the parameters already used by guaranteed containers.
The goal is to reduce the number of parameters that need to be used by the user.

We also fix a small issue related to the container metrics (opportunistic memory reported in GB in Web UI, although it was in MB).