Add support for new function GenerateSshRsaPrivateKey

We already support [GenerateSshRsaPublicKey](https://github.com/terraform-ibm-modules/ibmcloud-terratest-wrapper/blob/ef865987502384a916cc889ba61b79ecdeba3960/common/general.go#L202), so can we update the code to also support `GenerateSshRsaPrivateKey` ?