Fix cluster CI: remove duplicate primary INSERT and add sync_algorithm=3

- Remove redundant INSERT of primary into proxysql_servers on nodes.
  Commit b02147b92 added the primary to the shared PROXYSQL_SERVERS_SQL
  but didn't remove the pre-existing per-node INSERT, causing a UNIQUE
  constraint violation that broke all multi-node CI groups.

- Add cluster_mysql_servers_sync_algorithm=3 to test_cluster_sync config
  files and update hardcoded datadir paths to match current workspace.

test/infra/control/start-proxysql-isolated.bash

@@ -212,8 +212,6 @@ SET admin-restapi_port=${RESTAPI_PORT};
 SET admin-restapi_enabled='true';
 SET admin-debug='true';
 ${PROXYSQL_SERVERS_SQL}
--- Also add the primary
-INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES ('proxysql',6032,0,'primary');
 LOAD ADMIN VARIABLES TO RUNTIME;
 SAVE ADMIN VARIABLES TO DISK;
 LOAD PROXYSQL SERVERS TO RUNTIME;

test/tap/tests/test_cluster_sync_config/test_cluster_sync_nomonitor/test_cluster_sync.cnf

@@ -1,4 +1,4 @@
-datadir = "/home/rene/proxysql/test/tap/tests/test_cluster_sync_config/test_cluster_sync_nomonitor";
+datadir = "/data/rene/proxysql2/proxysql/test/tap/tests/test_cluster_sync_config/test_cluster_sync_nomonitor";
 admin_variables : 
 {
   admin_credentials = "admin:admin;radmin:radmin";
@@ -16,6 +16,7 @@ admin_variables :
   cluster_mysql_users_diffs_before_sync = 3;
   cluster_admin_variables_diffs_before_sync = 3;
   cluster_proxysql_servers_diffs_before_sync = 3;
+  cluster_mysql_servers_sync_algorithm = 3;
   cluster_sync_interfaces = false;
 };
 mysql_variables : 

test/tap/tests/test_cluster_sync_config/test_cluster_sync_withmonitor/test_cluster_sync.cnf

@@ -1,4 +1,4 @@
-datadir = "/home/rene/proxysql/test/tap/tests/test_cluster_sync_config/test_cluster_sync_withmonitor";
+datadir = "/data/rene/proxysql2/proxysql/test/tap/tests/test_cluster_sync_config/test_cluster_sync_withmonitor";
 admin_variables : 
 {
   admin_credentials = "admin:admin;radmin:radmin";
@@ -16,6 +16,7 @@ admin_variables :
   cluster_mysql_users_diffs_before_sync = 3;
   cluster_admin_variables_diffs_before_sync = 3;
   cluster_proxysql_servers_diffs_before_sync = 3;
+  cluster_mysql_servers_sync_algorithm = 3;
   cluster_sync_interfaces = false;
 };
 mysql_variables :