Create globally scoped Vault client

[patoarvizu]

Currently, the operator creates a new Vault client on every reconcile loop. Instead, a global client instance should be created at the beginning of the lifecycle of the operator, and reuse on every reconcile action.

Note that because the lifetime of the operator is likely to be longer than the TTL of any token the client gets, the operator should handle token refresh (or reauthentication if refresh is not possible).

Ideally, the solution for this should keep #55 in mind, to consider the possibility of needing to maintain global clients for multiple endpoints.

[patoarvizu]

Done in #58