Auto-generate SSH/SSL secrets on first boot

[arm4b]

There are concerns about security keys generated during the CI process and so considered insecure.
And while Vagrant is considered as a solution for demo purposes, OVA could be used in prod environments.

The following needs to be auto-generated on first VM boot:

• SSH host keys (see related https://github.com/StackStorm/ova/issues/21)
• Stanley private SSH key & take care of Authorized key
• SSL certificate used for nginx/st2web

Solution would be systemd configs which rely on ssh/nginx services (runs as "before" dependency) and creates keys/certs if they're not there already.
This will ensure that SSH/SSL secrets are unique and not the same across all the users after our Packer builds.

Apart of that #17, - need to document as insecure:

• vagrant Linux user with default vagrant password
• vagrant Linux user with default/known Hashicorp SSH private key

[arm4b]

BTW, this issue is actual for any pre-packaged artifact, incl. Docker, see:

https://github.com/StackStorm/st2-docker/blob/d00781776c17e43774940f472fa9929ea432be76/images/stackstorm/Dockerfile#L107-L110

https://github.com/StackStorm/st2-docker/blob/d00781776c17e43774940f472fa9929ea432be76/images/stackstorm/Dockerfile#L127

While it's easy to fix in VM, it'll a bit harder to workaround in Docker.