contint: +libmysqlclient-dev package on labs slaves

When building the python module SQLAlchemy from pip, we end up with:

    sh: 1: mysql_config: not found

We just need the MySQL client dev package which provides the utility.

Change-Id: I232ed4aa20fc466e4594902401b2d4f68dce3050
Merge "contint: +libmysqlclient-dev package on labs slaves" into production