Refactored fetcher code in containerizers.

Review: https://reviews.apache.org/r/27995
Made ProcessBase::route hard-fail rather than soft fail.

It is an API expectation that the path start with '/', and if it doesn't
it should be a programmer error.

No code actually checked the return code. If a developer did get this
wrong previously it would silently do nothing and in debugging their
endpoint just wouldn't appear. This makes it simple/easy for them to
find.

Review: https://reviews.apache.org/r/27660
Added task labels to mesos.proto.

Review: https://reviews.apache.org/r/28154
Added task labels modeling to http helpers.

Review: https://reviews.apache.org/r/28155
Added task labels to internal task message.

Review: https://reviews.apache.org/r/28156
Wired up task labels in master.

Review: https://reviews.apache.org/r/28157
Wired up task labels in slave.

Review: https://reviews.apache.org/r/28158
Added label to ModelTask test.

ModelTask needed to be updated to parse new labels field.

Review: https://reviews.apache.org/r/28188
Match future dispatch messages with type info.

Review: https://reviews.apache.org/r/28052
Fix future dispatch in tests to match function type.

Review: https://reviews.apache.org/r/28081