This commit moves Write function to separate yaml package, and makes
it available to anyone for dumping documents in single file, with
correct yaml separators '---', '...'.
This will allow, for example dumping of filtered from bundle documents
to disk or any writer, like Stdout, for debugging purposes. Specifically
it will be used as part of `airshipctl cluster initinfra` command as a
buffer for delivering resources to kubernetes cluster.
Change-Id: I780e3f2d2ff446b8787153f500d04d10487ed71b
f8f6f8be27