properly convert proxy config from install.inf (bnc#885957)

Originally Yast did not change the `"enabled"` `Proxy` value which resulted in writing `PROXY_ENABLED="no"` to `/etc/sysconfig/proxy` and not writing `/root/.curlrc` file.

Fixes:

- set `enabled` to `true` when importing `Proxy` config
- use `URI` for parsing URL
- do not log the proxy password to `y2log`
- added test case