Add mysql devel as testonly alongside mysql-server

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
diff --git a/files/apts/neutron b/files/apts/neutron
index d3a08c7..23dd65b 100644
--- a/files/apts/neutron
+++ b/files/apts/neutron
@@ -2,6 +2,7 @@
 iptables
 iputils-ping
 iputils-arping
+libmysqlclient-dev  # testonly
 mysql-server #NOPRIME
 sudo
 python-boto