Add mTLS option for redfish test

When the BMC only enables mTLS auth type at build time. Robot test
should be test normally. Adding "MTLS_ENABLED" environment variable as
an option and modifications to achieve it.

Signed-off-by: Tony Lee <tony.lee@quantatw.com>
Change-Id: I68c753a453f0c958c900f8ba3e1d73145e093d6c
diff --git a/lib/resource.robot b/lib/resource.robot
index 9d5379e..91d81bb 100755
--- a/lib/resource.robot
+++ b/lib/resource.robot
@@ -21,6 +21,14 @@
 ${OPENBMC_PASSWORD}    0penBmc
 ${REST_USERNAME}       root
 ${REST_PASSWORD}       0penBmc
+
+# MTLS_ENABLED indicates whether mTLS is enabled.
+${MTLS_ENABLED}        False
+# Valid mTLS certificate for authentication.
+${VALID_CERT}          ${EMPTY}
+# Path of mTLS certificates directory.
+${CERT_DIR_PATH}       ${EMPTY}
+
 ${IPMI_PASSWORD}       0penBmc
 ${MACHINE_TYPE}    palmetto
 ${DBUS_POLL_INTERVAL}      15s