devstack doesn't check ASTARA_APPLIANCE_SSH_PUBLIC_KEY existence

[zhang sheng]

When use devstack to install astara. Astara will never check the existence of the ASTARA_APPLIANCE_SSH_PUBLIC_KEY.

In current code, we use mode "source" to check the existence of the ssh public key.
https://github.com/openstack/astara/blob/master/devstack/plugin.sh#L343

According to the devstack plugin reference, There are no "source" mode when devstack call plugin.

So, We should use the mode "stack" and phase "pre-install" to check prerequisites.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/264589
Committed: https://git.openstack.org/cgit/openstack/astara/commit/?id=35ffb2cf5170d612a4a98b9f8cb1091604ba25c6
Submitter: Jenkins
Branch: master

commit 35ffb2cf5170d612a4a98b9f8cb1091604ba25c6
Author: zhang sheng <email address hidden>
Date: Thu Jan 7 14:16:35 2016 +0800

    devstack doesn't check ASTARA_APPLIANCE_SSH_PUBLIC_KEY existence

    Change plugin from "source" to mode "stack" and phase "pre-install" to generate the default ssh keypair.

    Change-Id: Ie52095db3342cfa01114a2fd1c67220b9edf9626
    Closes-Bug: #1531723