Feature add docker/volume and volume/secret support (#58)

[JustinVenus]

Description:

Implement `docker/volume` and `volume/secret` support on the scheduler and python client.

* update docs for using docker/volume and volume/secret

* implement scheduler changes for docker/volume and volume/secret

* implement python thrift changes for docker/volume and volume secret

* update python thrift tests for docker/volume and volume/secret

* update aurora client schema for docker/volume and secret/volume

* create/update structs for docker/volume and volume/secret isolators

Testing Done:

This patch was originally applied to rel/0.21.0 and has been updated to apply to the current master branch. In the form applied to rel/0.21.0 it has been running in my production environments since December 2018.

As far as how to test the new features.

docker/volume

I have dvdcli-0.2.0. I run rexray for EBS support. The mesos-agent uses AWS IAM to describe/attach/detach/etc EBS volumes.

# /etc/rexray/config.yml
libstorage:
  service: ebs
  integration:
    volume:
      operations:
        mount:
          preempt: true
        remove:
          force: true
        create:
          default:
            size: 2
            type: gp2
            fsType: xfs
  server:
    services:
      ebs:
        driver: ebs
ebs:
  maxRetries:         10
  statusMaxAttempts:  10
  statusInitialDelay: 100ms
  statusTimeout:      60s

# /etc/systemd/system/multi-user.target.wants/rexray.service
[Unit]
Description=REX-Ray Storage Orchestration Engine
After=network-online.target cloud-config.target cloud-init-local.service

[Service]
Environment=DOCKER_LEGACY=true
Restart=always
ExecStart=/usr/bin/rexray service start -f

[Install]
WantedBy=multi-user.target

volume/secret

This is a little more interesting to test. I'm not aware of any publicly available secret resolver implementations. At minimum you have to write your own secret resolver. We chose to target with hashicorp/vault and implent a kv2 resolver.