Support for client certificat

[jerome-jutteau]

Is your feature request related to a problem?/Why is this needed

Some deployments need to use a specific client certificate in order to make calls.
For instance, use may want to configure its API Access Rules with certiticates

/feature

The feature would consist to pass an optional client certificate to use through environment variable to CSI.
In a Kubernetes deployment, this certificate should be stored inside a Secret.

Two variables can be configured:

• OSC_CLIENT_CERTIFICATE (client-certificate-data) base64(PEM certificate)
• OSC_CLIENT_KEY (client-key-data): base64(PEM RSA private key)

Both variables should be set or none of them. When both are set, CSI will use client-certificate-data and client-key-data to establish connexion to Outscale API.