Provide support for deploying apps from enterprise github

[cwb124]

Detailed Description

Would love to be able to push apps via stratos from our private enterprise github. Right now it seems as if only public github is available.

Context

Many enterprises have their own github instance to deploy to their on-prem clouds. Think this would be beneficial to them rather than deploying from a file

Possible Implementation

Either add another source type within the Deploy Application page or better yet change the box on the right labeled Public Git URL to Any valid Git repo URL, or something like that and add a place to authenticate to enterprise github.

[irfanhabib]

@cwb124 Deploying using the Public Git URL source type should allow you to deploy from a private GitHub enterprise repository.
Support for private repositories is something we plan to do in the near future.

[cwb124]

Stratos is lacking a way to authenticate to an enterprise github repo. I've added that to the implementation above.

[richard-cox]

@cwb124 Thanks for raising this issue. We previously had quite a cool way to handle github token's via the endpoints page. Bringing this back in would enabled us to deploy apps from private/enterprise github. There's a similar request in #1442, but asking for more basic credential management to use with non-github repos. Ideally we should tackle this at the same time.
In terms of implementing the feature we'd like to tackle this in our new v2 but are first working towards parity with v1.