Hide [share] image_password from log

Fix the missing secret attribute to hide the sensitive credential in
logs.

Trivial-Fix

Change-Id: Ieec26d31791e07f400fc8d5ef3e47704891e46e0
diff --git a/manila_tempest_tests/config.py b/manila_tempest_tests/config.py
index dc16100..511d86f 100644
--- a/manila_tempest_tests/config.py
+++ b/manila_tempest_tests/config.py
@@ -308,6 +308,7 @@
                default="manila",
                help="Image username."),
     cfg.StrOpt("image_password",
+               secret=True,
                help="Image password. Should be used for "
                     "'image_with_share_tools' without Nova Metadata support."),
     cfg.StrOpt("client_vm_flavor_ref",