AuthProxy: Allow disabling Auth Proxy cache (#83755)

* extract auth proxy settings

* simplify auth proxy methods

* add doc mentions

docs/sources/setup-grafana/configure-security/configure-authentication/auth-proxy/index.md

@@ -36,7 +36,8 @@ header_property = username
 auto_sign_up = true
 # Define cache time to live in minutes
 # If combined with Grafana LDAP integration it is also the sync interval
-sync_ttl = 60
+# Set to 0 to always fetch and sync the latest user data
+sync_ttl = 15
 # Limit where auth proxy requests come from by configuring a list of IP addresses.
 # This can be used to prevent users spoofing the X-WEBAUTH-USER header.
 # Example `whitelist = 192.168.1.1, 192.168.1.0/24, 2001::23, 2001::0/120`
@@ -231,6 +232,10 @@ ProxyPassReverse / http://grafana:3000/
 
 With our Grafana and Apache containers running, you can now connect to http://localhost/ and log in using the username/password we created in the htpasswd file.
 
+{{% admonition type="note" %}}
+If the user is deleted from Grafana, the user will be not be able to login and resync until after the `sync_ttl` has expired.
+{{% /admonition %}}
+
 ### Team Sync (Enterprise only)
 
 > Only available in Grafana Enterprise v6.3+