This function combines calls to the sftpConnect, sftpPutFile, and
sftpDisconnect functions.
A random string will be prefixed to the file name to prevent overwriting existing files on the
server.
sftpUploadFile(privateKeyFileName, userName, remoteFolder = ".", fileName)A character string denoting the path to an RSA private key.
A character string containing the user name.
The remote folder to upload the file to.
A character string denoting the path to file to upload.