Add support for docker storage options

Docker driver plugin

Co-Authored-By: nickethier <ncethier@gmail.com>

Vault API Client Tweaks

Keep attempting to renew Vault token past locally recorded expiry, just
in case the token was renewed out of band, e.g. on another Nomad server,
until Vault returns an unrecoverable error.

Return Vault TTL info to /agent/self API and `nomad agent-info` command.

Seems like the stats field is a micro-optimization that doesn't justify
the complexity it introduces.  Removing it and computing the stats from
revoking field directly.

Add a gauge to track remaining time-to-live, duration of renewal request API call.

Add CircleCI configuration for website build

docs for affinity

Fix a panic related to batch GC

Vendor update consul template

vault: fix panic by checking for nil secret

Fix typo in GNUmakefile