browser_session - Make sure that base_url is reset when things go wrong

There is a wrong base_url present in ``conf.env['base_url']`` when vm_analysis test fails inside a ``with browser_session(base_url=<ip_address>):``

see: https://url.corp.redhat.com/7386c31

I'm pretty sure it's because the code after the yield statement is not executed and this should fix it.