Change default values to follow ansible community form of 'yes' or 'no'

Also change the sd_alert_cleanup to be the safer 'no' as default, as wiping existing alerts should be explicit.

defaults/main.yml

 ---
 # file: roles/serverdensity/defaults/main.yml
 
-sd_update_remote: true
+sd_update_remote: yes
 sd_url: ''
 sd_api_token: ''
-sd_api_cache_file: false
+sd_api_cache_file: no
 sd_agent_key: ''
 sd_logging_level: 'info'
-sd_alert_cleanup: true
-sd_force_update: true
+sd_alert_cleanup: no
+sd_force_update: yes
 sd_plugins: []
 sd_groups:
   apache: 'none'