Pull integration of Solum from an external Git repo

This blueprint targets the functionality to pull source code from an external Git repository into Solum.

Proposed workflow:
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Solum will provide a API endpoint against which users would be able to do a POST with url of an existing Github repository in the request body. Solum will perform appropriate checks on the specified url, and if they pass, will clone the repository and store it for further processing.

Work Items (Adding them here as the Work Items section seems not to be working for me):
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Following work items are envisioned:

1) Define and implement the appropriate API resource

2) Define and implement a mechanism to allow specification and enforcement of security rules for the Github repository urls. Some of the example rules could be:
- allowed structure of url (e.g.: ^http://github.com/[a-z/-]+$)
- whitelisted urls
- blacklisted urls

3) Investigate and implement the functionality of cloning the specified git repository
     3.1) Investigation piece 1: Whether Github's API can be used for this purpose or do we need to use git cli wrapped in Fabric for this.
     3.2) Investigation piece 2: Where do we store the cloned git repo? We need this so that steps further in the workflow can use it for actions such as detecting the language pack required, compiling, packaging, etc. Two options that jump out are:
    - storing the repo in some temporary known location on the API server
    - storing the repo in a Swift container