vinyl: abort bootstrap if vinyl directory is not empty

If the user sets snap_dir to an empty directory by mistake while leaving
vinyl_dir the same, tarantool will still bootstrap, but there is likely
to be errors like:

  vinyl.c:835 E> 512/0: dump failed: file './512/0/00000000000000000001.run' already exists
  vy_log.c:1095 E> failed to rotate metadata log: file './00000000000000000005.vylog' already exists

Even worse, it may eventually fail to restart with:

  vy_log.c:886 E> ER_MISSING_SNAPSHOT: Can't find snapshot

To avoid that, let's check the vinyl_dir on bootstrap and abort if it
contains vylog files left from previous setups.

Closes #2872