Acceptance Tests: move useful ssh methods to base class

Acceptance Tests: move useful ssh methods to base class

Both the virtiofs submounts and the linux ssh mips malta tests
contains useful methods related to ssh that deserve to be made
available to other tests.  Let's move them to an auxiliary, mix-in
class that will be used on the base LinuxTest class.

The method that helps with setting up an ssh connection will now
support both key and password based authentication, defaulting to key
based.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210412044644.55083-5-crosa@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>