Fix metadata, update base image version.

[paulbouwer]

This PR updates the container image metadata to include the version value in the org.label-schema.docker.cmd label and also correctly render the ${HOME} variable in that label.

Existing:

PS> docker image inspect azuresdk/azure-cli-python:2.0.20 --format "{{json .Config.Labels}}" | ConvertFrom-Json | ConvertTo-Json
{
    "org.label-schema.build-date":  "2017-10-19T18:26:52Z",
    "org.label-schema.description":  "The Azure CLI 2.0 is the new Azure CLI and is applicable when you use the Resource Manager deployment model.",
    "org.label-schema.docker.cmd":  "docker run -v :/root -it azuresdk/azure-cli-python:\u003cversion\u003e",
    "org.label-schema.license":  "MIT",
    "org.label-schema.name":  "Azure CLI 2.0",
    "org.label-schema.schema-version":  "1.0",
    "org.label-schema.url":  "https://docs.microsoft.com/en-us/cli/azure/overview",
    "org.label-schema.usage":  "https://docs.microsoft.com/en-us/cli/azure/install-az-cli2#docker",
    "org.label-schema.vcs-url":  "https://github.com/Azure/azure-cli.git",
    "org.label-schema.vendor":  "Microsoft",
    "org.label-schema.version":  "2.0.20"
}

Updated:

PS> docker image inspect azuresdk/azure-cli-python:2.0.20-test --format "{{json .Config.Labels}}" | ConvertFrom-Json | ConvertTo-Json
{
    "maintainer":  "Microsoft",
    "org.label-schema.build-date":  "2017-10-26T00:38:26Z",
    "org.label-schema.description":  "The Azure CLI 2.0 is the new Azure CLI and is applicable when you use the Resource Manager deployment model.",
    "org.label-schema.docker.cmd":  "docker run -v ${HOME}/.azure:/root/.azure -it azuresdk/azure-cli-python:2.0.20-test",
    "org.label-schema.license":  "MIT",
    "org.label-schema.name":  "Azure CLI 2.0",
    "org.label-schema.schema-version":  "1.0",
    "org.label-schema.url":  "https://docs.microsoft.com/en-us/cli/azure/overview",
    "org.label-schema.usage":  "https://docs.microsoft.com/en-us/cli/azure/install-az-cli2#docker",
    "org.label-schema.vcs-url":  "https://github.com/Azure/azure-cli.git",
    "org.label-schema.vendor":  "Microsoft",
    "org.label-schema.version":  "2.0.20-test"
}

This PR also updates the container base image version to python:3.6.3-alpine.