cloudapi: add option to upload S3 artifacts publicly

A small test is added to cover this new option. Also, a check for AWS region
in the URL was removed. The public URL doesn't actually contain it and it's
imho useless - S3 buckets are always tied to a certain region.

The elaborate grep command parsing a path from a URI was switched to being
a small python script. The python script can actually handle an URI without
a query (which was always present in the pre-signed URI, but it's not in the
ordinary one)

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/cloudapi/v2/openapi.v2.yml

@@ -844,6 +844,16 @@ components:
         region:
           type: string
           example: 'eu-west-1'
+        public:
+          type: boolean
+          default: false
+          description: |
+            If set to false (the default value), a long, obfuscated URL
+            is returned. Its expiration might be sooner than for other upload
+            targets.
+            
+            If set to true, a shorter URL is returned and
+            its expiration is the same as for the other upload targets.
     GCPUploadOptions:
       type: object
       required: