fix: return default bucket acl if none exists

[benmcclelland]

We were trying to parse a non existing acl and returning an internal server error due to invalid json acl data.

If the bucket acl does not exist, return a default acl with the root account as the owner.

This fixes #1060, but does not address the invalid acl format from s3cmd reported in #963.

[niksis02]

Do we really need to implement the GetRootAccount method in iam ?
If you store the root access key in fiber context it could be retreived in AclParser and passed to ParceACL function, which woulde handle the check for empty acl input and return the default ACL with root user access.

I think it makes sense to isolate this acl behavior in ParseACL in the AclParser middleware ?

[benmcclelland]

Do we really need to implement the GetRootAccount method in iam ? If you store the root access key in fiber context it could be retreived in AclParser and passed to ParceACL function, which woulde handle the check for empty acl input and return the default ACL with root user access.

I think it makes sense to isolate this acl behavior in ParseACL in the AclParser middleware ?

I'll take a look, I think I had some issue with only the iam service getting the root account details. Maybe we could pass this to the s3api too though.