Add convinient way to configure root URL for binary artefacts

[nizhikov]

Corporate environment often restricts internet access.
So, to provide access to yamake binary artefacts from mapping.conf.json (https://devtools-registry.s3.yandex.net) one can substitute every URL in the file to the corporate proxy.

But, it will be more convenient to have configuration for a root URL like the following.
So, only root url must be changed in ext_mapping.conf.json:

{
    "servers": {
        "main": "https://devtools-registry.s3.yandex.net"
    },
    "bottles": {},
    "resources": {
        "6277415836": "{main}/6277415836",
        "2214720943": "{main}/2214720943",
        "2214542408": "{main}/2214542408",
        "1886578148": "{main}/1886578148",
...
}