gencert.py: Generate each users's certificate at scratch_link.py start

The script gencert.sh prepares the certificate for the WSS server.
The certificate is stored in the bluepy-scratch-link directory path, and
shared by users on the machine. This certificate sharing among users is
not handy in case one user prepares bluepy-scratch-link and another user
use it, since the latter user may not know where the certificate is.

Address this issue by generating the certificate for each user.
Re-implement gencert.sh as a python module gencert.py, and integrate it
in scratch_link.py. With this, when a user starts scratch_link.py,
the certificate unique for the user is generated and stored under user
home directory at the path "~/.local/share/blupy-scrach-link".

The newly added python module gencert.py depends on pyOpenSSL to generate
the certificates. Add this dependency to requirements.txt.

Signed-off-by: Shin'ichiro Kawasaki <kawasaki@juno.dti.ne.jp>

scratch_link.py

@@ -26,6 +26,9 @@ import threading
 import time
 import queue
 
+# for websockets certificate
+import gencert
+
 logLevel = logging.INFO
 
 # handle command line options
@@ -642,10 +645,13 @@ class BLESession(Session):
             self.delegate.restart_notification_event.set()
         return self.status == self.DONE
 
+# Prepare certificate of the WSS server
+gencert.prep_cert()
+
 # kick start WSS server
 ssl_context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
-localhost_cer = pathlib.Path(__file__).with_name("scratch-device-manager.cer")
-localhost_key = pathlib.Path(__file__).with_name("scratch-device-manager.key")
+localhost_cer = gencert.cert_file_path
+localhost_key = gencert.key_file_path
 ssl_context.load_cert_chain(localhost_cer, localhost_key)
 sessionTypes = { '/scratch/ble': BLESession, '/scratch/bt': BTSession }