Neutron needs to be able to install MySQL-python in the functional
test job, which requires devstack to install mysql devel libraries as
testonly.
As per sdague's recommendation mysql devel libraries are now installed
for nova as well. In the future, any service that installs
mysql-server should also install mysql devel libraries for consistency.
Related-bug: #1346444
Change-Id: Ie715841e33a2c2d31cb4ac724b302eb98ed3366a