Fix: Remove Turn errors on Coturn startup

[pielonet]

Hi,

This is a fix for Coturn default configuration set in bbb-install-2.6.sh:

• TLS and DTLS for Coturn is unneeded locally and must be disabled (TLS termination is done by haproxy)

0: WARNING: cannot find certificate file: turn_server_cert.pem (1)
0: WARNING: cannot start TLS and DTLS listeners because certificate file is not set properly
0: WARNING: cannot find private key file: turn_server_pkey.pem (1)
0: WARNING: cannot start TLS and DTLS listeners because private key file is not set properly

• "lt-cred-mech" is incompatible with "use-auth-secret" so removing it

CONFIGURATION ALERT: You specified --lt-cred-mech and --use-auth-secret in the same time.
Be aware that you could not mix the username/password and the shared secret based auth methohds. 
Shared secret overrides username/password based auth method. Check your configuration!

• "cli-password" need not be empty although not used

CONFIG ERROR: Empty cli-password, and so telnet cli interface is disabled! Please set a non empty cli-password!

[pielonet]

Also BBB 2.6 default Turn configuration (/etc/bigbluebutton/turn-stun-servers.xml) should be modified to only propose turns (TLS over port 443/TCP) and disable unencrypted configuration for port 3478.