Enhance coverage for builder trigger-builds

Some options to the trigger-builds builder were not present in the test
fixture.  As a side effect that enhance our documentation.

Change-Id: Ic2755f76f02f7eba3842dc9a41f7db8907acda75
Added config options to not overwrite jobs desc

When you do not specify any description for a job, right now it creates a
new one overwriting whatever you had put there manually. Now when
setting the configuration option 'keep_descriptions' to True, it will
only overwrite the description if you specified one explicitly in the
yaml.
That way you can avoid specifying any description and it will not
overwrite the ones you put there manually through the jenkins ui.

Added a couple of tests for it also, and extended the tests to allow custom
configuration by adding a *.conf file for it

Change-Id: I408f5ee06a6939a8cb8f4f2e6c6e0d060462259c
Signed-off-by: David Caro <dcaroest@redhat.com>
Merge "Added config options to not overwrite jobs desc"
Merge "Enhance coverage for builder trigger-builds"