build/base: fix systemd connection to dbus

It seems that dbus and systemd don't agree on socket location.
While the bug is fixed upstream, let's create a symlink to have systemd
working.

Bug reported in Red Hat:
https://bugzilla.redhat.com/show_bug.cgi?id=1130939

This patch may be reverted once the bug fixed upstream or somewhere else
in eDeploy roles.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #121 from enovance/systemd-dbus

build/base: fix systemd connection to dbus on RHEL7