Allow to issue Vault token with admin policy

Summary:
Ops need to authenticate to Vault to manage PKI, credentials, mounts, policies.
This is covered by the 'admin' policy.

To facilitate the authentication, this script allows each ops member to issue
such token as a self-service facility.

Usage: sudo utils/vault/issue-admin-token.py [--insecure]

The --insecure argument allows to use this script even if the TLS certificate
has expired. That's required to issue through Vault a new certificate.

Ref T1975

Test Plan:
Tested on Complector with sudo utils/vault/issue-admin-token.py
by Dorian and me to get our new tokens.

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1975

Differential Revision: https://devcentral.nasqueron.org/D3357