Remove MariaDB-shared in openstack-base

After we switched to PyMySQL in openstack-base, there is no need to
compile MySQL-python thus to install MariaDB-shared package as
a workaround for busted percona packages.

TrivialFix
Related-Bug: #1526129

Change-Id: I93b4bee9d42427b493dac3a0039e0b1a9cac9f71