Consolidates the addition of the fetcher container to the pod into a new type FetcherConfig and takes care of serviceAccountName if not set. Also added PreStop lifecycle handler if pod set TerminationGracePeriodSeconds
This commit solves part of the issue #52 , functions are able to access secrets/configmaps specified by the user. For now, CLI only accept one secret/configmap. For advanced users, it will be able to use YAML to declare multiple secrets/configmaps in later changes.