Update LCOW tests

[rn]

• Update the daemon start. No env variables needed anymore
• Use --platform linux where needed
• Add a test which pulls with --platform linux and then runs without --platform linux
• Add a few tests for docker image save/load/export

With the latest docker master build:

docker version
Client:
 Version:       master-dockerproject-2018-02-14
 API version:   1.36
 Go version:    go1.9.4
 Git commit:    3e5f2f29
 Built: Wed Feb 14 23:50:11 2018
 OS/Arch:       windows/amd64
 Experimental:  false
 Orchestrator:  swarm

Server:
 Engine:
  Version:      master-dockerproject-2018-02-14
  API version:  1.36 (minimum version 1.24)
  Go version:   go1.9.4
  Git commit:   68c3201
  Built:        Wed Feb 14 23:57:03 2018
  OS/Arch:      windows/amd64
  Experimental: true

all tests pass, except:

[FAIL    ] lcow.image.save 2.10s
[FAIL    ] lcow.image.export 2.17s
[FAIL    ] lcow.build.multistage_scratch 27.18s
[FAIL    ] lcow.network.localhost 1.69s