ws: Expose host key for CockpitSshTransport

This allows us to look at the host key that the server offered,
regardless of whether verification succeeded or failed. Via
cockpit_ssh_transport_get_host_key() and the 'host-key' property.

In addition we want to be able to tell CockpitSshTransport to
explicitly expect a certain key and ignore other host verification
via the known-hosts file. This happens via 'host-key' construct
property.
ws: Modernize how test-webservice uses test fixtures

Use a struct for the fixture so we can extend them. In addition use
TestCase for the test struct, so we can be less confused.
ws: Expose the host-key in the close channel message

If a channel closes with reason "unknown-hostkey" try to include
the host-key and host-fingerprint in the message so it can be
acted on by the caller.

I tried to come up with a better way to do this, such as allow
us to discover the host key for successfully opened channels. The
alternatives were complex and had all sorts of corner cases.

This solves the immediate need. The concept of having more
close fields is also used in the upcoming spawn-channel code
to relay exit codes and exit signals. So this bit is sound.
ws: Add a channel open 'host-key' field for expecting a key

If we know which key to expect (such as after the user has
approved it) we can expect just that host-key when opening
a channel to a remote machine.
web: Expose the error details in DBusClient

After closing due to an unknown-hostkey failure we want the details
of the host-key and host-fingerprint to be accessible on the
DBusClient.
daemon: Make Machines object testable and add basic test

Simplify the periphery and around Machines and Machine objects
and decouple them from Daemon. This allows us to start writing
tests for this stuff. In addition construct properties don't
need to be getable if that's not necessary functionality.

Add a single test of a basic 'Add' call to Machines. This is
not meant to be comprehensive, but just get us started.
daemon: Implement support for host keys when adding machine

Add a new host_key parameter to Machines.Add(). If non-empty the
key there gets added to our known_hosts file.
web: Fix 'Login with my current credentials' checkbox

This checkbox had no effect.
ws: Use display MD5 ssh key fingerprints as ssh client does

Use the standard MD5 fingerprints for display, just like the
openssh ssh client does.
ws: Quiet down mock-ssh a bit when it gets killed
web: Display fingerprints in initial setup and add to known_hosts

Show real host fingerprint in initial setup and add to known_hosts
so that we can later connect to the host without problems.

We refactor the connectting to another server in PageSetupServer a
bit since we need to reconnect at various stages with different
credentials and keys.

Closes #468