Acceptance Tests: set up existing ssh keys by default

Acceptance Tests: set up existing ssh keys by default

It's questionable whether it's necessary to create one brand new pair
for each test.  It's not questionable that it takes less time and
resources to just use the keys available at "tests/keys" that exist
for that exact reason.

If a location for the public key is not given explicitly, the
LinuxTest will now set up the existing pair of keys as the default.
This removes the need for a lot of boilerplate code.

To avoid the ssh client from erroring on permission issues, a
directory with restrictive permissions is created for the private key.
This should still be a lot cheaper than creating a new key.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210203172357.1422425-19-crosa@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[marcandre: fix typos in commit message]
Signed-off-by: Cleber Rosa <crosa@redhat.com>