Implement --plain-http as a configuration

[SteveLasker]

For local testing and IoT scenarios that use plain HTTP, we currently need to add --plain-http as a parameter.

• notation sign --plain-http $IMAGE
• notation pull --plain-http --peek $IMAGE
• notation verify --plain-http $IMAGE

Alternatively, what if we added insecureRegistries as a configuration option:

"insecureRegistries": [
    "registry.wabbit-networks.io"
  ]

Expected Behavior

• notation sign $IMAGE
• notation --list $IMAGE
• notation verify $IMAGE

@shizhMSFT

[shizhMSFT]

Will be included in the next iteration.

[shizhMSFT]

Resolved by 07dada7

[SteveLasker]

Resolved with the 0.5.2 release