remove container complete callback functionality

poll GetContainer instead

[#79618102]
namespace executor-specified properties
container can be created with tags
container is fully specified at allocation time

this removes the data split between alloc + init, which will make later
refactoring easier

[#79618102]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
restore callback functionality for now

...so that we can push our changes up iteratively without having to
complete the whole thing first

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>