contint: include browsertest role on labs slaves

Any labs slave should be able to run browser tests. So include the role
in the labs::common class.

In eqiad we need /mnt to be mounted (via role::ci::slave::labs::common).
Make sure the browsertests class depends on it.

Harness openjdk packages which are also defined by the elasticsearch
module.

Change-Id: I121a256dcbd5ab1e4e7503e81a74ee32b63882a0