Use mergeDeviceWithConfiguration from IOTA Lib instead of defining own function

[fgalan]

While reviewing PR #640 we have realized that while IOTA-JSON uses directly the mergeDeviceWithConfiguration function from library:

https://github.com/telefonicaid/iotagent-json/blob/master/lib/iotaUtils.js#L246

In the case of IOTA-UL the same fucntion is redefined within lib/iotaUtils.js (in fact that's why PR #640 is necessary).

This is weird... IOTA-UL should aling with IOTA-JSON and use also the function directly from library (thus simplifying and gainging the advantages of code reuse).

Maybe there is a good reason (that we don't remember now :) to do the current way, in which case an explination should be provided as comment in this issue before closing it.

[fgalan]

PR #644