Make the stack runnable from default config (#4445)

When running the stack after copying the default configuration file, the
stack won't start because of errors due to fake paths defined for
firebase and encryption.
Let's comment them to ease incoming developers' life.

cozy.example.yaml

@@ -56,9 +56,9 @@ admin:
 # to generate the keys
 vault:
   # the path to the key used to encrypt credentials
-  credentials_encryptor_key: /path/to/key.enc
+  # credentials_encryptor_key: /path/to/key.enc
   # the path to the key used to decrypt credentials
-  credentials_decryptor_key: /path/to/key.dec
+  # credentials_decryptor_key: /path/to/key.dec
 
 # file system parameters
 fs:
@@ -371,7 +371,7 @@ notifications:
   development: false
 
   # Firebase Cloud Messaging API
-  fcm_credentials_file: /etc/cozy/fcm_credentials.json
+  # fcm_credentials_file: /etc/cozy/fcm_credentials.json
 
   # APNS/2 certificates for iOS notifications
   # ios_certificate_key_path: path/to/certificate.p12