Handle NApp requirement

[diraol]

How to deal with the NApp requirements.txt file, speacially regarding kytos package version.

[cemsbr]

@diraol, maybe we have already discussed this: python packages in README and NApp deps in kytos.json.

[diraol]

@cemsbr I think that we defined that non-python packages dependencies should be documented on the NApp README. But for python packages this is not strictly necessary, since we can install the NApp python dependencies by checking the NApp requirements.txt file. May be we can install them by just using pip to do it.

[cemsbr]

That would be easy if there were only virtual environments. However, the way it is now, it is possible to have Kytos installed by root and a user willing to install a NApp, with all the corner cases.

Besides, think about a NApp using version x of a Python package and another using version y. In such cases, only one virtualenv per NApp could solve the problem.

For now, a quick and (sometimes) useful solution is to try pip install -r requirements.txt like @diraol suggested, and warn the user if it fails for any reason (permission, compilation, etc).

What do you think, @beraldoleal?