Removed unnecessary 'whitelisted' variable in the allocator.

Review: https://reviews.apache.org/r/28662
Cleaned up the utility structs in the allocator.

Review: https://reviews.apache.org/r/28663
Updated Allocator::slaveAdded to take the total resources explicitly.

Review: https://reviews.apache.org/r/28665
Updated allocator.hpp to idiomatically use the "Process wrapper" pattern.

Review: https://reviews.apache.org/r/28666
Cleaned up the allocator method naming.

Review: https://reviews.apache.org/r/28667
Fixed a bug in the allocator's framework activation.

Review: https://reviews.apache.org/r/28668
Updated allocator to only check initialization in exposed methods.

Review: https://reviews.apache.org/r/28669
Fixed a compiler warning from clang.
Splitted resource and resource usage checkers.

Review: https://reviews.apache.org/r/28618
Added basic DiskInfo check in master.

Review: https://reviews.apache.org/r/28626
Added tests for basic DiskInfo checker.

Review: https://reviews.apache.org/r/28627
Added duplicated persistence id check in ResourceChecker.

Review: https://reviews.apache.org/r/28664
Renamed task and offer visitors to validators.

Review: https://reviews.apache.org/r/28684
Moved repeated labels to Labels message.

We suggested to change hooks into manipulating task labels only. With
that in mind and based on BenH's suggestion originally, this patch set
wraps the repeated Labels field in a message when can be used without
referring to repeated field types.

Review: https://reviews.apache.org/r/28574
Changed task to use Labels.

Review: https://reviews.apache.org/r/28575
Changed HTTP task modeling to use Labels.

Review: https://reviews.apache.org/r/28576
Changed tests to use Labels.

Review: https://reviews.apache.org/r/28577
Add slave id to docker container name prefix.
Add option to disable docker containerizer killing orphans
Add executor for docker containerizer