Download a certificate from the server and save it to a specified file path.
The path argument should include the file name and extension where the certificate will be stored.
For example, '/path/to/certificate.crt'. The recommended file extension for certificates is '.crt'.

alpacon cert download [CERT ID] [flags]

	alpacon cert download [CERT ID] --out=/path/to/certificate.crt

  -h, --help         help for download
  -o, --out string   path where certificate should be stored