Support for shared configuration in pipelines

[sandrogattuso]

This change should allow to set contexts in a pipeline resource. I've compiled a version of the provider from my branch and tested it with a secret context and a config context. The pipeline created loads correctly the contexts.

I've updated the test files, however, I couldn't get the test working locally since I'm getting this error (NOTE: I get the below even from master branch without any change, so this is not related to the changes introduced in this PR):

 testing_new.go:106: terraform failed: exit status 1
        stderr:
        Error: Failed to instantiate provider "codefresh" to obtain schema: Unrecognized remote plugin message: 

I'm happy to perform further validations and have the test cases validated on my end, but I might need some guidance on how to resolve the above.

Closes #15

[palson-cf]

@sandrogattuso
Thanks a lot for contributing.

I've tested your changes and everything seems working.

Regarding testing - you're right. Tests didn't work.
It would be better to divide contexts and tests on two separate PRs. But let leave it as is.
I found a few broken tests as well. Will fix them after your merge.

Please add the new context attribute to the pipelines docs and I will merge your changes.

[sandrogattuso]

@palson-cf updated the pipeline.md and pipelines.md (in examples folder). Let me know if I've missed anything else or if you want me to squash commits (not sure what's the guideline on this repo in that regard)