mysql: Don't bother to change auth plugin on centos

This partially reverts the previous change [1], which
broke networking-midonet jobs.

[1] https://review.opendev.org/#/c/681201/

Closes-Bug: #1855516
Change-Id: I0255c6acce72a8376dbc6d8f8d0314a7dabf019c
1 file changed