[Openstack] Add importer for openstack

[imnitishng]

This is a draft pull request for adding Openstack advisories aimed to close #374

The data importer currently handles only fetching the packages
from openstack and corresponding versions of the packages using
GithubTags.

Further iterations aim at fetching vulnerabilities and adding tests.
However, I'd like to have this reviewed and discussed before moving further.

[sbs2001]

@imnitishng this looks good at first glance. Except for the usage of async. Keep it simple use requests, I don't think using async here has much value here wrt the complexity it brings.

You'd need to fix the dco.

For help with tests and other issues you're facing use the chat at https://gitter.im/aboutcode-org/vulnerablecode

[sbs2001]

@imnitishng ping

[imnitishng]

@sbs2001 very sorry for the delay, I have been a bit occupied lately I plan to complete this very soon.

[sbs2001]

@imnitishng no worries, was just making sure you were not facing some issue :)

[pombredanne]

@imnitishng ping ... do you think you may complete this? Otherwise I will close it for now. Thanks

[Hritik14]

@imnitishng I have been looking at this importer and could assist on the code if you face any problem at all.

[imnitishng]

I'm a bit occupied so I can't work on this @Hritik14 you can take this up if you want or @pombredanne can close it.

[pombredanne]

In light of #374 (comment) I am closing this for now and we can reopen when times come to work on this.
Thank you ++ for your efforts!