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