Stop setting "RequestTTY force" for SSH

[pjcdawkins]

• Adds a configuration setting ssh.options allowing arbitrary SSH options to be appended.
• Adds an option --option (-o) in the ssh command for additional options.
• Passes on options to SSH without validating or changing them.
• Stops setting the "RequestTTY force" option for SSH in the ssh command.
  This change is because this option is potentially irritating for scripting users and unnecessary for others.
  The previous behavior can be restored with either:
  • -o 'RequestTTY force' or
  • export PLATFORMSH_CLI_SSH_OPTIONS='RequestTTY force'
• Allow SSH to continue even after failing to write configuration files.
• Do not (re)write config when running ssh-cert:load --refresh-only.