Set the OS_TEST_DBAPI_ADMIN_CONNECTION override variable so that
oslo.db opportunistic detection will know to use PyMySQL until
I12b32dc097a121bd43991bc38dd4d289b65e86c1 makes it the default
behavior.
This change removes previous code[1] enabling PyMySQL use.
[1] Ic5c1d12ab75443e1cc290a7447eeb4b452b4a9dd
Change-Id: Ic39feee0248f2ffabdba26f3779ab40a8f3838e6 Co-Authored-By: Victor Sergeyev <vsergeyev@mirantis.com> Co-Authored-By: Cedric Brandily <zzelle@gmail.com>