Refactor storage of allocated and garden containers

[#79618102]

Signed-off-by: Max Brunsfeld <mbrunsfeld@pivotal.io>
keep usage in sync with garden

Signed-off-by: Max Brunsfeld <mbrunsfeld@pivotal.io>
if listing containers fails, don't give up hope

log loudly

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>