Add TLS for mysql

Use ssl_mode for mysql and add docs
add docs for the new parameters in config

Tolerate ssl_mode without client authentication

Client cert is not necessary for a SSL connection. So we tolerate
failure if client cert is not provided.
Improve error message if missing server_cert_name and mode is not
skip-verify.

conf/defaults.ini

@@ -63,9 +63,15 @@ name = grafana
 user = root
 password =
 
-# For "postgres" only, either "disable", "require" or "verify-full"
+# For "postgres", use either "disable", "require" or "verify-full"
+# For "mysql", use either "true", "false", or "skip-verify".
 ssl_mode = disable
 
+ca_cert_path =
+client_key_path =
+client_cert_path =
+server_cert_name =
+
 # For "sqlite3" only, path relative to data_path setting
 path = grafana.db