Add documentation for eligibility toggle endpoint

Rebased docs reorg 

Link Enterprise in header and footer to Nomad Enterprise docs page instead of marketing page; add "request more info" links

Initial device go-plugin

UI: Specialized No Leader error page

Fix kill timeout exceeding 5m on Docker driver

Fixes an issue where the Docker API client would timeout before the kill
timeout was hit.

Clarify `max_parallel` interaction with taskgroups

Updates are parallelized at the task group level always.  The
`max_parallel` attribute controls parallelization over the `count`
allocations of the task group itself.

Error if TLS Certificate signature algorithm isn't supported in cipher suites

Vendor github.com/golang/protobuf/ptypes/empty