enhance testing.conftest

* fail early if there is no running selenium-standalone
* webserver is now a session scoped fixture
use yaml.safe_load in the credital loader
update .gitignore
kill RC api and do some invasive test refactorings
record deletion of the rc client tests
kill rcclient from sauce labs
move parts of the selenium client setup to a fixture

this kills the need for a teardown hook
own plugin for safety featues + fixture changes
move the base url testing to a autouse fixture
replace baseurl from mozwebqa.cfg with selenium_base_url in pytest ini config
some flake8 fixes
turn the safety into a mostly own plugin
code cleanups in plugin_safety
html reporting to own plugin
make testing webserver a daemon thread
remove the unimplemented network capture option
simplify saucelab cappabilities
better code for pytest keywords to saucelabs tags
add forgotten return
shorten proxy config code