reset registration URL when registration upgrade fails (bnc#891293)

fallback to SCC to have the same behavior like in full installation
removed the hardcoded submission config, use the defaults
added test
changes

- 3.1.128
fixed test (mock instead of a temporary change)
do not shadow a local variable

removed conflict between block argument and a local variable
Merge pull request #174 from yast/reset_url_after_failed_upgrade

Reset URL after failed upgrade to allow switch from SMT to SCC