Make upload-results task optional

[averevki]

Closes #27

[azgabur]

https://tekton.dev/docs/pipelines/pipelines/#param-enum I would really like to use the enum type here as there will be just two allowed values "true" and "false" but this feature is still in alpha and not supported on Openshift without some modification. Otherwise looking good, lgtm!

[trepel]

LGTM, I would improve description to explicitly state that this is meant to be boolean value (despite actually being of string type).
And I would also consider saying that everything except for "true" is treated as false.

But that's minor stuff, approving