checkbox-touch: fix startup with no test plans

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
checkbox-touch: rename providers_collection to provider_list

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
checkbox-touch: actually load regular providers

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
checkbox-touch: load and add the built-in category provider

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/checkbox-touch-fixes/ by tarmac [r=kissiel][bug=][author=zkrynicki]"