Automate GitLab Geo installation process

Right now the installation for GitLab Geo is a bit cumbersome. On multiple nodes, you have to setup custom PostgreSQL options, SSH keys, GitLab admin settings, etc.: https://docs.gitlab.com/ee/gitlab-geo/configuration.html

We should strive to make the installation really simple for users. For example, DataStax OpsCenter makes it really easy to add a single Cassandra node by giving the SSH credentials necessary to login as root (e.g. see https://docs.datastax.com/en/opscenter/5.1/opsc/online_help/opscAddingNode_t.html). All the setup happens automatically after that.

/cc: @regisF, @brodock, @rspeicher, @dbalexandre

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Goal for %9.1: We decided to implement the proposed gitlab-ctl commands: add-geo-primary-node, add-geo-secondary-node and try to get done as many "steps" we can.

Some of the steps are challenging to what we have available as APIs and infrastructure, so we may make a simpler implementation that may not automate fully (or just print more detailed/precise instructions, like: copy this content to Admin > ...)