Config's box.cfg applier scans snapshot_dir in order to find out,
whether recovery is going to be done. It's needed in order to determine,
whether the instance should be started into ro mode firstly.

Let's move info about snapshot into separate file in utils. The commit
also introduces snapshot_path, which will be used in the following
commits in order to validate names.

Needed for #8978

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring