tests/api: custom repository tests

- remove `custom-repos.sh` integratoin test
- add custom repositories check to `api` tests for supported
  images
- verify custom repositores are added to /etc/yum.repos.d
- verify gpg key is saved to /etc/pki/rpm-gpg (for inline keys)

test/cases/api/aws.sh

@@ -63,7 +63,7 @@ function createReqFile() {
         "name": "user2",
         "key": "$(cat "${WORKDIR}/usertest.pub")"
       }
-    ]${SUBSCRIPTION_BLOCK}${DIR_FILES_CUSTOMIZATION_BLOCK}
+    ]${SUBSCRIPTION_BLOCK}${DIR_FILES_CUSTOMIZATION_BLOCK}${REPOSITORY_CUSTOMIZATION_BLOCK}
   },
   "image_request": {
       "architecture": "$ARCH",