Add ability to skip for test_signed_image_upload_boot_snapshot

when environment doesn't have an ability to sing image,
this test will fail, for this case
added test skip condition.

Change-Id: I426e0eb9e94386578d5ee2e756edd128a2dd9f0a
Related-prod: PRODX-362
(cherry picked from commit c272618c8bcca3172aedfe38205e88e563ffeb90)
1 file changed