Stop using deprecated intermediate certificates

[fcharlier]

Lecm currently concatenates an intermediate cert lets-encrypt-r3-cross-signed.pem which is deprecated and expires on September 29, 2021.

In addition, Let's Encrypt now returns the full chain along with the certificate, adding hardcoded intermediate is not even necessary anymore.

This change does the following:

• stop adding the intermediate certificate to the cert
• stop writing the intermediate certificate file
• split the chained certificates into a <name>-chain.pem file for software which still need separate chain certificates in a separate file.

Also use PyYaml SafeLoader instead of FullLoader to avoid warnings & support older releases of PyYaml.

[sbadia]

Oh thanks @fcharlier for this !