server/server: exclude-plugin param in /health route

- Added a new querystring param to the /health route: `exclude-plugin`
  - Can be specified multiple times
  - Value is the name of a plugin that should be excluded from the OK
    status check performed when the `plugins` qs param is specified

Fixes #3713

Signed-off-by: Grant Shively <gshively@godaddy.com>