Allow per-repository endpoints

[tlrx]

Add the ability to define endpoint and protocol on a per-repository basis. This way, users can define multiple S3 repository that store data on different S3 providers.

$ curl -X PUT 'http://localhost:9200/_snapshot/test' -d '{
    "type":"s3",
    "settings":{
        "bucket":"test-bucket",
        "endpoint":"non.aws.endpoint.com",
        "access_key":"...",
        "secret_key":"..."
    }
}'

Related to #128

[tlrx]

Closed by d680b94eb748585ed7eebdce1c4184fe61829ad5