Support running Vault Agent cache in sidecar container

[lawliet89]

https://www.vaultproject.io/docs/agent/caching/index.html

[hamishforbes]

It would be helpful to have an annotation that enables a listener {} and cache {} block in the sidecar agent.

Something like

vault.hashicorp.com/agent-listen: bool (default: false)
vault.hashicorp.com/agent-listen-tls-disable: bool (default: true)
vault.hashicorp.com/agent-listen-address: string (default: 127.0.01:8200)
vault.hashicorp.com/agent-cache-auto-auth: bool (default: false)

I'm migrating a number of services onto k8s which previously were able to access Vault via an agent listening on plain HTTP bound to 127.0.0.1.

I can reproduce this in k8s but only by using the configmap method to add a listener, which is ok but very verbose and repetitive for many services

[lawliet89]

#132 implements this. But I have kept TLS always disabled for now because there is no way to mount arbitrary volumes right now.