Introduces Rancher's metadata service as an optional provider source for
Traefik, enabled by setting `rancher.MetadataService`.
The provider uses a long polling technique to watch the metadata service and
obtain near instantaneous updates. Alternatively it can be configured to poll
the metadata service every `rancher.RefreshSeconds` by setting
`rancher.MetadataPoll`.
The refactor splits API and metadata service code into separate source
files respectively, and specific configuration is deferred to
sub-structs.
Incorporates bugfix #1414
