Move shared code to testlib

Signed-off-by: Andrey Berezin <a.berezin@yadro.com>

src/frostfs_testlib/shell/ssh_shell.py

@@ -29,7 +29,7 @@ reporter = get_reporter()
 class HostIsNotAvailable(Exception):
     """Raised when host is not reachable via SSH connection."""
 
-    def __init__(self, host: str = None):
+    def __init__(self, host: Optional[str] = None):
         msg = f"Host {host} is not available"
         super().__init__(msg)