Deb: provide the default-mysql-* packages that exist in Debian/Ubuntu

Since Debian 9 (Stretch) and Ubuntu 16.10 (Yakkety) the following
packages have existed:
* virtual-mysql-client
* virtual-mysql-client-core
* virtual-mysql-server
* virtual-mysql-server-core
* virtual-libmysqlclient-dev

They are metapackages that in Debian depend on MariaDB and in
Ubuntu currently on MySQL. We need to provide them and point
them to MariaDB so that systems that have the mariadb.org
repositories enabled automatically get everything MariaDB
and not MySQL.

This change makes the packaging provide the four first ones,
and later commits will fix the client library issues.

debian/control

@@ -204,6 +204,7 @@ Provides: mysql-client-core,
           mysql-client-core-5.5,
           mysql-client-core-5.6,
           mysql-client-core-5.7,
+          default-mysql-client-core,
           virtual-mysql-client-core
 Description: MariaDB database core client binaries
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
@@ -256,6 +257,7 @@ Provides: mysql-client,
           mysql-client-5.5,
           mysql-client-5.6,
           mysql-client-5.7,
+          default-mysql-client,
           virtual-mysql-client
 Recommends: libdbd-mysql-perl (>= 1.2202), libdbi-perl, libterm-readkey-perl
 Description: MariaDB database client binaries
@@ -306,6 +308,7 @@ Provides: mysql-server-core,
           mysql-server-core-5.5,
           mysql-server-core-5.6,
           mysql-server-core-5.7,
+          default-mysql-server-core,
           virtual-mysql-server-core
 Description: MariaDB database core server files
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
@@ -376,7 +379,7 @@ Replaces: libmariadbclient-dev (<< 5.5.0),
           mysql-server-5.6,
           mysql-server-5.7,
           virtual-mysql-server
-Provides: virtual-mysql-server
+Provides: virtual-mysql-server, default-mysql-server
 Description: MariaDB database server binaries
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
  server. SQL (Structured Query Language) is the most popular database query