Fix console support with cells

The (relatively recent) security fixes for consoleauth do not work
with cells because consoleauth and the compute manager for the instance
talk to different rabbit servers.  consoleauth is in the API cell and
the compute manager is in a child cell.

This patch adds code to proxy the following via cells:

1) compute -> consoleauth's delete_tokens_for_instance()
2) consoleauth -> compute's validate_console_port()

Also: On instance deletion when the compute manager tells consoleauth to
delete tokens for the instance, it's not necessary to do an rpc.call and
block.  The only purpose it could serve would be log a traceback on failure
on the compute manager side, but that seems unnecessary.  There's no
return value.  This patch changes it to an rpc.cast instead.

Fixes bug 1154755

Change-Id: Ic763f2a5c0dcff7b7ccfac5927680e7881605f61
Merge "Fix console support with cells"