bump grootfs
bump grootfs

[#171848540]

Submodule src/grootfs 218ddf5d..217f094c:
  > Propagate the log timestamp format when re-execing

Co-authored-by: Giuseppe Capizzi <1968455d84ef56ceb93e8ec69a3034a6fe0ab9cc@pivotal.io>

cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release