Provide passphrases for client certificates in http resource

[SimonBaeumer]

Further reading:

• Decrypt pkcs8 https://github.com/youmark/pkcs8/blob/master/pkcs8.go#L103
• https://stackoverflow.com/questions/37316370/how-create-rsa-private-key-with-passphrase-in-golang
• https://golang.org/pkg/crypto/tls/#X509KeyPair