Add basic Savanna CLI tests

It contains basic, read-only tests for Savanna CLI client. Checks return values
and output of read-only commands. It tests the following commands:

* this-does-not-exist
* plugin-list
* plugin-show
* node-group-template-list
* cluster-template-list
* cluster-list

Change-Id: I67d3d1e6f72e5c4e07cdb26743d020cb1e91a434
Merge "Add basic Savanna CLI tests"