Enable generated app to accept REST server URLs for multiple business networks

[sstone1]

https://github.com/hyperledger/composer/issues/3802

Allow proxy configuration to accept an override for the REST server URL in two environments:

REST_SERVER_URL

This went in the first drop - a single URL override.

Example:

export REST_SERVER_URL=https://dogecorp.com:3000

REST_SERVER_URLS

This is new in this PR - a JSON string containing business network name to URL mappings for multiple business networks - useful for generic deployment scripts. The application knows which business network that it should look for.

Example:

export REST_SERVER_URLS='{
    "basic-sample-network": "https://basic-sample-network.dogecorp.com",
    "vehicle-manufacture-network": "https://vehicle-manufacture-network.simoncorp.com"
}'