Move install responsibilities to domU

As we are moving forward to test XenAPI in the gate, it is necessary to
move dom0 related modifications to be performed from domU. For this
purpose, a new user is created, and that user should be used to talk to
dom0 from domU. This change creates that user, makes it possible for
dom0 to log in to domU with that account, and configure that account to
be able to talk down to dom0.

Also move several steps to the nova xenserver plugin:
 - dom0 plugin installation
 - create kernels and images directory
 - install console rotate script
 - configure a cron to execute console rotate script

Configuration changes:
    A new configuration option, DOMZERO_USER has been created, that
    specifies a user account that is configured to be able to do
    passwordless ssh to dom0.

Change-Id: If9de0b297a67b7cdb5de78d8dd0e8b2ca578b601
Merge "Move install responsibilities to domU"