Add aliases for resources

[Serneum]

This adds aliases for cert, key, and secret resources. If an alias is specified, it is added to the appropriate map with the same value as the resource's name

Tested with the following config:

workers:
  -
    resources:
      - kind: secret
        name: secret-key-base
        alias: skb
        vaultBaseURL: https://vault-tst.vault.azure.net/
        credential: request-keys

    sinks:
      - path: secrets.json
        template: "{{ index .Secrets.skb.Value }}"
    frequency: 60s

Closes #56

[chrisjohnson]

I don't think it's necessary for this PR but it could make sense to add an aliases key for all-AllSecretsKind so a map of name => alias could be supplied. Maybe toss an issue to circle back to this work later.

Just to confirm, if you omit the alias key, it doesn't cause any errors right?

[Serneum]

@chrisjohnson correct

workers:
  -
    resources:
      - kind: all-secrets
        vaultBaseURL: https://vault1-tst.vault.azure.net/
        credential: cred1
      - kind: secret
        name: secret-key-base
        vaultBaseURL: https://vault2-tst.vault.azure.net/
        credential: cred2

    sinks:
      - path: secrets.json
        template: '{{ (index .Secrets "secret-key-base").Value }}'

works fine