dbconfig-common autopkgtest broken by mariadb-10.1

Hi,
it seems current autopkgtests of dbconfig-common are broken.

An example https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-artful/artful/amd64/d/dbconfig-common/20180524_161642_46223@/log.gz

I came by triggering this from a postgresql proposed upload to Artful, but after looking into it I think this is mysql/mariadb related.

The error usually is tests breaking on:
  ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2 "No such file or directory")

Not the post-install race but after reconfiguring it by the test.

I ran this locally in a lxd container and the error reproduces without anything from proposed.

I wondered about the mariadb versions.
The last goo test is with: mariadb-10.1/10.1.28-1
But in the archvie atm is a Security fix at 10.1.30-0ubuntu0.17.10.1

This was not tested in autopkgtest infrastructure and might be the actual breaking change, but that has to be tested in detail.

To reproduce
1. get a artful lxd container
2. get all dependencies for dbconfig-common tests
3. run the tests

All non mariadb tests will work, those installing mariadb instead of mysql will fail.

Note: given the ~6 weeks Artful has left there might be better places to spend time on, but other SRUs that touch this should not be blocked on it.
Unfortunately we can't mask mariadb-test as it breaks other packages tests.