Attribute enableServiceLinks can be now configured in PodSpec so that environment variables are not injected in function pods and DNS is used for discovery
The merge function executor used wasn't merge container correctly, and it
didn't merge all fields in spec except volumeMount & Env which confused people.
To apply the user-configured container correctly, this PR changes the way of merge
and follows rules:
1. Slices are merged and return an error if the elements in the slice have name conflicts.
2. Maps are merged, the value of map of dst container are overridden if the key is the same.
3. The rest of the fields of dst container are overridden directly.
