roles: strict truthy values

According to Ansible we can use yes, true, True, "or any quoted st-
ring" for a boolean true, but ansible-lint wants us to use either
true or false.

See: https://chronicler.tech/red-hat-ansible-yes-no-and/

web.yml

@@ -3,7 +3,7 @@
 
 - name: Configure web servers
   hosts: web
-  become: yes
+  become: true
   roles:
     - common
     - { role: mariadb, when: mariadb_databases is defined}