Add AppRole Authentication

[crayfishx]

This PR adds AppRole authentication to the vault_lookup function. I've tried to do this in a similar way to the current implementation of certs (although some of it is a bit convulted) to keep consistency. The lookup will still default to cert auth if nothing changes so it shouldnt pose any kind of backwards compatibility issues..... env vars can be overridden to enable approle auth.....

export VAULT_AUTH_METHOD=approle
export VAULT_ROLE_ID=XXXXX-XXXX-XXX-XX-XXXXXXXXXX
export VAULT_SECRET_ID=YYYYY-YYYY-YYY-YY-YYYYYYYYYYY

I simplified some of the function calls to avoid duplication of the token request code after adding this in.