Allow sourcing apt_key from URL

Allow deployers to specificy a remote URL to download apt signing
keys from.

Example:
galera_client_gpg_keys:
  - key_name: 'mariadb'
    url: "https://some.webserver.com/mariadb.gpg"
    fallback_url: "https://other.webserver.com/mariadb.gpg"
    hash_id: '0xcbcb082a1bb943db'
  - key_name: 'percona-xtrabackup'
    keyserver: 'hkp://keyserver.ubuntu.com:80'
    fallback_keyserver: 'hkp://p80.pool.sks-keyservers.net:80'
    hash_id: '0x1c4cbdcdcd2efd2a'

Change-Id: I781cb8f5744c4e1e8e728a8ad308d135d2e5922c

tasks/haproxy_add_ppa_repo.yml

@@ -41,6 +41,7 @@
     id: "{{ item.hash_id }}"
     keyserver: "{{ item.keyserver | default(omit) }}"
     data: "{{ item.data | default(omit) }}"
+    url: "{{ item.url | default(omit) }}"
     state: "present"
   register: add_keys
   until: add_keys|success
@@ -54,14 +55,15 @@
 - name: Add haproxy apt-keys using fallback keyserver
   apt_key:
     id: "{{ item.hash_id }}"
-    keyserver: "{{ item.fallback_keyserver }}"
+    keyserver: "{{ item.fallback_keyserver | default(omit) }}"
+    url: "{{ item.fallback_url | default(omit) }}"
     state: "present"
   register: add_keys_fallback
   until: add_keys_fallback|success
   retries: 5
   delay: 2
   with_items: haproxy_gpg_keys
-  when: add_keys|failed and item.fallback_keyserver is defined
+  when: add_keys|failed and (item.fallback_keyserver is defined or item.fallback_url is defined)
   tags:
     - haproxy-apt-keys