bump grootfs
bump grootfs

Submodule src/grootfs 886e347..7d7266a:
  > Rename ImageCloner to ImageManager

Co-authored-by: Georgi Sabev <cd7643e50243a10153b49cb72b4bdbf8b48cb045@gmail.com>

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