Move playbooks to root directory (fixes #2513)

Some time ago a "playbooks" subdirectory had been created and setup.yml
had been moved there (as matrix.yml), together with the new
jitsi_jvb.yml. However, jitsi_jvb.yml wouldn't work at all, unless a
symbolic link to it was created from the root directory. A symbolic link
setup.yml for playbooks/matrix.yml had already been created and
committed.

This seems to indicate that the developers thought they should place all
playbooks in a subdirectory, but when this didn't work, they linked to
one of them, leaving the other one inoperative.  We fix this by moving
the playbooks to the root directory. This works, and the pollution of
the root directory is not too bad since it's only two playbooks.

We also change the mode of setup.yml to 644 (it had inexplicably changed
to 755 in 9735a2f600).

This contribution is provided by GRNET S.A. (grnet.gr).

jitsi_jvb.yml

@@ -0,0 +1,12 @@
+---
+- name: "Set up additional Jitsi JVB servers"
+  hosts: "jitsi_jvb_servers"
+  become: true
+
+  roles:
+    - role: galaxy/com.devture.ansible.role.playbook_help
+    - role: galaxy/com.devture.ansible.role.systemd_docker_base
+
+    - custom/matrix-base
+    - galaxy/jitsi
+    - custom/matrix-common-after