Add Keepalive command

[ngetahun]

How to review

# build the sle15sp3 docker image
> docker build -t connect.15sp3 -f Dockerfile.15sp3 .

# Run the container
>docker run --privileged --network=host --rm -it connect.15sp3 bash

# Run the glue server in another terminal
> cd happy-customer/glue && bin/rails s

# Scenario 1: Un-registered system
# in the container, run
> bin/SUSEConnect --keepalive
# You should see an error message, directing you to register the container

# Scenario 2: Register system from regcodes in scc/glue
> bin/SUSEConnect -r xxxxx --url http://localhost....

> bin/SUSEConnect --keepalive

# Visit the glue web ui. Under the org > systems view, you should see the last_seen_at attribute updated

# Scenario 3: Simulate a pub-cloud machine
# On a registered system
> touch /usr/sbin/registercloudguest
> bin/SUSEConnect --keepalive

# You should see a success message.

Related trello card: https://trello.com/c/NFqpY5Vt/2272-uras-suseconnect-keepalive-subcommand

[ngetahun]

[URAS] SUSEConnect --keepalive subcommand

[suse-tests-pass]

Well Done! Your tests are still passing.
https://ci.suse.de/job/SCC-Connect-Prophet/479985/console
If the given link has expired,you can force a Prophet rerun by just deleting this comment. (Merged 02d62af into fbad4c6)