mariadb: write data directory sentinel file

More robust in case mysql_install_db fails but creates the directory.

Note this will require to manually create the sentinel file for data
directories already bootstrapped.
2 jobs for mariadb-init-sentinel in 3 minutes and 56 seconds (queued for 28 minutes and 14 seconds)
Status Job ID Name Coverage
  Test
canceled #13820
ai3
run_base_test

failed #13821
ai3
run_buster_test

00:03:56

 
Name Stage Failure
failed
run_buster_test Test
base : Set locale to C -------------------------------------------------- 0.04s
ERROR: Command '['ansible-playbook', '-i', 'config.yml', 'site.yml']' returned non-zero exit status 2
Running playbook site.yml...
ERROR: failed to run the site.yml playbook
~/builds/x_qLRQwE/0/ai3/float/test/test-buster ~/builds/x_qLRQwE/0/ai3/float/test/test-buster ~/builds/x_qLRQwE/0/ai3/float/test
==> host1: Removing domain...
~/builds/x_qLRQwE/0/ai3/float/test/test-buster ~/builds/x_qLRQwE/0/ai3/float/test
No ControlPath specified for "-O" command
ERROR: Job failed: exit status 1