s3/client: Keep server port on config

Currently the code temporarily assumes that the endpoint port is 9000.
This is what tests' local minio is started with. This patch keeps the
port number on endpoint config and makes test get the port number from
minio starting code via environment.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

test/boost/s3_test.cc

@@ -22,6 +22,7 @@
 
 s3::endpoint_config_ptr make_minio_config() {
     s3::endpoint_config cfg = {
+        .port = std::stoul(tests::getenv_safe("S3_SERVER_PORT_FOR_TEST")),
     };
     return make_lw_shared<s3::endpoint_config>(std::move(cfg));
 }