1070585: GUI no longer locks on connection test. Adds cancel button.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1070585

Hit some cases like user entering:
example.com
example.com:345
http://example.com
http://example.com:1232
user@example.com:2138
user:password@example.com
http://user@example.com:2138
example.com:
:2138

It doesn't necessarily have to understand those variants (some are just wrong), but
need to verify it handles the errors. rhsm.utils.get_ent_proxy_info or rhsm.utils.parse_url
may be useful, if not a touch of overkill.