Use Ansible dig lookup instead of calling the dig program

This means we no longer have a dependency on the `dig` program,
but we do have a dependency on `dnspython`.

Improves things as suggested in #65 (Github issue).

roles/matrix-server/tasks/self_check/self_check_dns.yml

@@ -1,24 +1,23 @@
 ---
 
+# This requires the dnspython library and will fail with a friendly error when unavailable.
 - name: Check DNS SRV record
-  shell:
-    cmd: "dig -t srv {{ ('_matrix._tcp.' + hostname_identity + '.')|quote }}"
-  register: result_dig_srv
-  changed_when: false
-  ignore_errors: true
+  set_fact:
+    result_dig_srv: "{{ lookup('dig', ('_matrix._tcp.' + hostname_identity + './SRV'), 'flat=0', wantlist=False) }}"
 
-- name: Fail if dig failed
+- name: Fail if DNS SRV record missing
   fail:
-    msg: "Failed checking DNS SRV record. You likely don't have the `dig` program installed locally. Full error: {{ result_dig_srv }}"
-  when: "result_dig_srv.stderr != ''"
+    msg: "It appears the DNS SRV record for {{ hostname_identity }} is not set up correctly (the record is missing). See the 'Configuring DNS' documentation for this playbook."
+  when: "result_dig_srv == 'NXDOMAIN'"
 
-# We expect an answer like this:
-# ;; ANSWER SECTION:
-# _matrix._tcp.DOMAIN. 10800	IN	SRV	10 0 8448 matrix.DOMAIN.
 - name: Fail if DNS SRV record incorrect
   fail:
-    msg: "It appears the DNS SRV record for {{ hostname_identity }} is not set up correctly. See the 'Configuring DNS' documentation for this playbook. Full DNS answer was: {{ result_dig_srv.stdout }}"
-  when: "('8448 ' + hostname_matrix) not in result_dig_srv.stdout"
+    msg: >
+      It appears the DNS SRV record for {{ hostname_identity }} is not set up correctly.
+      Expected it to point to `{{ (hostname_matrix + '.') }}` (port 8448).
+      Found it pointing to `{{ result_dig_srv.target }}` (port {{ result_dig_srv.port }}).
+      See the 'Configuring DNS' documentation for this playbook.
+  when: "result_dig_srv.target != (hostname_matrix + '.') or result_dig_srv.port != 8448"
 
 - name: Report correct DNS SRV record
   debug: