Private key for user hadoop in vanilla plugin.

Write cluster-wide private key and authorized_keys for user hadoop in
the Vanilla plugin.
This is the same key we use in
instances.py#_generate_user_data_script().

There's going to be corresponding fix in savanna dib elements.

Partially fixes: bug #1188442
Implements: blueprint avoid-keypair-in-dib-scripts

Change-Id: I81b98053d4176d7f87bf9ed1b7e64eea6994562d