Enable creating a rancher client by directly passing a *Config

[git-ival]

Issue: N/A

Problem

It is not currently possible to create a rancher Client without relying on Shepherd's configuration file load mechanism which generates Config structs as needed.

Solution

Small refactor to the rancher client's client.go which pulls out the majority of the client creation logic into a separate function and adds the NewClientForConfig function which takes a *Config directly.

Testing

Tested locally within the rancher/dartboard project, but have not run any existing tests elsewhere.

Regressions Considerations

There should be no regressions as the flow of the NewClient function has not been modified.

Backports:

• [BP 2.9] enable creating a rancher client by directly passing a *Config #311
• [BP 2.8] Enable creating a rancher client by directly passing a *Config #312

[caliskanugur]

Left a nit, looks good Iramis

Could you create a r/r PR with DNM for integration tests, and provide a few basic freeform runs offline?

We probably will need backports for this