use hashicorp lib for az cli auth support

[drewby08]

Using https://github.com/hashicorp/go-azure-sdk to implement az cli based authentication.
Added new CredentialConfig CliAuth which adds a az cli authenticated client to our map of clients.
Supports naming the az cli authenticated client just like any other Credential.

credentials:
  -
    name: cli
    cliAuth: true
  -  
    name: spn
    tenantID: my-tenant-id
    clientID: my-client-id
    clientSecret: my-client-secret

workers:
  -
    resources:
      - kind: secret
        name: mysecret
        vaultBaseURL: https://my-key-vault.vault.azure.net/
        credential: cli
    frequency: 5s
    sinks:
      - path: ./password
        template: "{{ .Secrets.mysecret.Value }} using cli"
  -
    resources:
      - kind: secret
        name: mysecret
        vaultBaseURL: https://my-key-vault.vault.azure.net/
        credential: spn
    frequency: 5s
    sinks:
      - path: ./password2
        template: "{{ .Secrets.mysecret.Value }} using spn"

[drewby08]

Haven't gotten this working yet

[drewby08]

Relying on original implementation for SPN based auth right now.