Support environment variable config substitutions when BRUBECK_CONFIG_SUBSTITUTE_ENV is set

[esmet]

I propose adding support for environment variable config substitutions, using a bask-like syntax, to obviate the need for any external config template solutions when using brubeck in a container deployment, where environment variables are the norm.

This feature is toggled via BRUBECK_CONFIG_SUBSTITUTE_ENV so users may opt-in to this behavior, since any config using this syntax is not technically valid json and could theoretically be a breaking change, though unlikely in practice.

A simple local test case does the right thing, and is valgrind-clean:

$ GRAPHITE_SERVER=foo GRAPHITE_PORT=1234 BRUBECK_CONFIG_SUBSTITUTE_ENV=1 valgrind ./brubeck --conf brubeck.json
==21044== Memcheck, a memory error detector
==21044== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==21044== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==21044== Command: ./brubeck --conf brubeck.json
==21044==
instance=(null) replaced config GRAPHITE_SERVER with foo from environment
instance=(null) replaced config GRAPHITE_PORT with 1234 from environment
[FATAL]: failed to resolve address 'foo'
==21044==
==21044== HEAP SUMMARY:
==21044==     in use at exit: 1,064,806 bytes in 206 blocks
==21044==   total heap usage: 1,316 allocs, 1,110 frees, 1,152,072 bytes allocated
==21044==
==21044== LEAK SUMMARY:
==21044==    definitely lost: 776 bytes in 1 blocks
==21044==    indirectly lost: 4,930 bytes in 95 blocks
==21044==      possibly lost: 1,232 bytes in 35 blocks
==21044==    still reachable: 1,057,868 bytes in 75 blocks
==21044==         suppressed: 0 bytes in 0 blocks
==21044== Rerun with --leak-check=full to see details of leaked memory
==21044==
==21044== For counts of detected and suppressed errors, rerun with: -v
==21044== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

and

$ cat brubeck.json
{
    "sharding" : false,
    "server_name" : "brubeck",
    "dumpfile" : "/var/log/brubeck/brubeck.dump",
    "capacity" : 15,
    "expire" : 2147483647,
    "http" : ":8080",

    "backends" : [
        {
            "type" : "carbon",
            "address" : "${GRAPHITE_SERVER}",
            "port" : ${GRAPHITE_PORT},
            "frequency" : 60
        }
    ],

    "samplers" : [
        {
            "type" : "statsd",
            "address" : "0.0.0.0",
            "port" : 8126,
            "workers" : 4,
            "multisock" : true,
            "multimsg" : 1
        }
    ]
}