Basic build slave needs to be CentOS 6

OpenJDK 1.8.0 isn't available until EL 7.1 finally comes out. As such we
need to keep using EL6 for now so that we can offer up Java8 to
projects.

Change-Id: I2cd94fb5b6281f826c4d70a806867af2f1af5dab
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>
Merge "Basic build slave needs to be CentOS 6"