roles/nginx: Separate Ubuntu 16.04 "Xenial" Let's Encrypt deps

Instead of iterating over fifteen packages with a loop that does fifteen
separate apt transactions, it is better to give the apt module a list so
it can install them all in one transaction. This is both quicker and te-
chnically more safe for dependency resolution.

roles/nginx/defaults/main.yml

@@ -30,6 +30,46 @@ letsencrypt_certbot_dest: /opt/certbot-auto
 # mainline is 1.15.x
 nginx_version: mainline
 
+# Dependencies of certbot-auto on Ubuntu 16.04 "xenial"
+# taken after running certbot-auto on a clean install
+letsencrypt_ubuntu_xenial_deps:
+  - augeas-doc
+  - augeas-tools
+  - binutils
+  - cpp
+  - cpp-5
+  - dialog
+  - gcc
+  - gcc-5
+  - libasan2
+  - libatomic1
+  - libcc1-0
+  - libcilkrts5
+  - libexpat1-dev
+  - libffi-dev
+  - libgcc-5-dev
+  - libgomp1
+  - libisl15
+  - libitm1
+  - liblsan0
+  - libmpc3
+  - libmpx0
+  - libpython-dev
+  - libpython2.7
+  - libpython2.7-dev
+  - libquadmath0
+  - libssl-dev
+  - libtsan0
+  - libubsan0
+  - python-dev
+  - python-pip-whl
+  - python-pkg-resources
+  - python-virtualenv
+  - python2.7-dev
+  - python3-virtualenv
+  - virtualenv
+  - zlib1g-dev
+
 # Dependencies of certbot-auto on Ubuntu 18.04 "bionic"
 # taken after running certbot-auto on a clean install
 letsencrypt_ubuntu_bionic_deps:

roles/nginx/tasks/letsencrypt.yml

@@ -13,49 +13,12 @@
 - name: Download certbot
   get_url: dest={{ letsencrypt_certbot_dest }} url=https://dl.eff.org/certbot-auto mode=700
 
-# dependencies certbot checks for on its first run
-# taken from running certbot right after a clean Ubuntu 16.04 install
 - name: Install certbot dependencies (Ubuntu 16.04)
   when: ansible_distribution == 'Ubuntu' and ansible_distribution_version is version_compare('16.04', '==')
-  apt: name={{ item }} state=present update_cache=yes
-  loop:
-    - augeas-doc
-    - augeas-tools
-    - binutils
-    - cpp
-    - cpp-5
-    - dialog
-    - gcc
-    - gcc-5
-    - libasan2
-    - libatomic1
-    - libcc1-0
-    - libcilkrts5
-    - libexpat1-dev
-    - libffi-dev
-    - libgcc-5-dev
-    - libgomp1
-    - libisl15
-    - libitm1
-    - liblsan0
-    - libmpc3
-    - libmpx0
-    - libpython-dev
-    - libpython2.7
-    - libpython2.7-dev
-    - libquadmath0
-    - libssl-dev
-    - libtsan0
-    - libubsan0
-    - python-dev
-    - python-pip-whl
-    - python-pkg-resources
-    - python-virtualenv
-    - python2.7-dev
-    - python3-virtualenv
-    - virtualenv
-    - zlib1g-dev
-  tags: packages
+  apt: name={{ letsencrypt_ubuntu_xenial_deps }} state=present update_cache=yes
+  tags:
+    - packages
+    - letsencrypt
 
 - name: Install certbot dependencies (Ubuntu 18.04)
   when: ansible_distribution == 'Ubuntu' and ansible_distribution_version is version_compare('18.04', '==')