repo+tests: Add [core]disable-xattrs=true, use it on overlayfs

repo+tests: Add [core]disable-xattrs=true, use it on overlayfs

There are a lot of things suboptimal about this approach, but
on the other hand we need to get our CI back up and running.

The basic approach is to - in the test suite, detect if we're on overlayfs. If
so, set a flag in the repo, which gets picked up by a few strategic places in
the core to turn on "ignore xattrs".

I also had to add a variant of this for the sysroot work.

The core problem here is while overlayfs will let us read and
see the SELinux labels, it won't let us write them.

Down the line, we should improve this so that we can selectively ignore e.g.
`security.*` attributes but not `user.*` say.

Closes: https://github.com/ostreedev/ostree/issues/758
Closes: #759
Approved by: jlebon