Parameterise token renewal intervals

[moertel]

For particular Vault setups, the default values for token renewals might
not be a good fit. This commit adds a parametrisation, so that the time
between a successful or failed token renewal and the next renewal attempt
can be controlled via optional environment variables.

VAULT_TOKEN_RENEWAL_INTERVAL:

• The time (in seconds) between a successful token renewal and the
  next renewal attempt. Default: 50% of the token lease duration

VAULT_TOKEN_RENEWAL_RETRY_INTERVAL:

• The time (in seconds) between a failed token renewal and the next
  renewal attempt. Default: 30 seconds

Refs #48

[moertel]

@ricoberger there you go. :) As the environment variables only make sense for token-based auth, I followed the style for configuring VAULT_TOKEN_LEASE_DURATION and added documentation to the Kubernetes secret as well as the Helm chart values but did not include it in the Helm templates themselves.

In order to maintain the previous renewal interval exactly, I made the values float instead of int.

[ricoberger]

Looks great. Thank you very much 🙂