Substitute ContainerBuildPlan for ContainerConfiguration and JibContainerBuilder.layerConfigurations

As mentioned in [JibContainerBuilder](https://github.com/GoogleContainerTools/jib/blob/26b5ff1440877458cb3a24a7c7b3799ce0bfa48c/jib-core/src/main/java/com/google/cloud/tools/jib/api/JibContainerBuilder.java#L77-L82).
```java
private final ContainerBuildPlan.Builder containerBuildPlanBuilder = ContainerBuildPlan.builder();
// TODO(chanseok): remove and use containerBuildPlanBuilder instead. Note that
// ContainerConfiguation implements equals() and hashCode(), so need to verify
// if they are required.
private final ContainerConfiguration.Builder containerConfigurationBuilder =
ContainerConfiguration.builder();
```

The two classes are basically identical. As of now, `JibContainerBuilder` is using both (i.e., duplicating configuration in each class): `ContainerConfiguration` for internal and historical use with the jib-core backend code, and `ContainerBuildPlan` for public API.

`layerConfigurations` can be removed too.
```
// TODO(chanseok): remove and use containerBuildPlanBuilder instead.
private List layerConfigurations = new ArrayList<>();
```