Allow to use private repository for default ChaosHub

[Calvinaud]

Hello,

The objective of this feature is to allow further customization of the default ChaosHub instead of only url/branch name https://github.com/litmuschaos/litmus/blob/master/chaoscenter/graphql/server/utils/variables.go#L32.

The idea was to allow to customize every field in https://github.com/litmuschaos/litmus/blob/master/chaoscenter/graphql/server/pkg/chaoshub/models_factory.go#L6

This feature will allow to use private repository for the ChaosHub that required authentication.

[djkormo]

To use litmus in disconnected environment when the only chaos hub is using a private repository this is an important problem.