Implement smarter merge mechanism

[nemesifier]

When two lists are merged with netjsonconfig.utils.merge_config, the result is the sum of the elements contained in the two lists.

However, often these lists are list of dictionaries representing interfaces and radios, and with the current implementation it is not possible to override the value of one of the dicts, this is best explained with an example.

We have a template in which radio0 has tx_power set to 20:

template = {
    "radios": [
        {
            "name": "radio0",
            "tx_power": 20
        }
    ]
}

And a configuration object in which radio0 has tx_power set to 10 (much lower than what's set in template):

config = {
    "radios": [
        {
            "name": "radio0",
            "tx_power": 10
        }
    ]
}

When we merge the two:

from netjsonconfig.utils import merge_config

result = merge_config(template, config)

result will look like the following:

config = {
    "radios": [
         {
            "name": "radio0",
            "tx_power": 20
        },
        {
            "name": "radio0",
            "tx_power": 10
        }
    ]
}

But what we want is:

config = {
    "radios": [
        {
            "name": "radio0",
            "tx_power": 10
        }
    ]
}

That is, we want to be able to override specifc attributes of dictionaries contained in lists.

[nemesifier]

The implementation introduced in f91b7b5 seems good enough.