fix: checking for internet connection can timeout

[gampnico]

In some CI environments or with low speed internet connections, HAS_INTERNET will return False due to a TimeoutError. This is a really rare bug. See DTCG logs here (requires authentication) and OGGM logs here.

This fix speeds up the connection time by avoiding a DNS lookup and application layer connection through HTTP. urlopen will also return a TimeoutError if the DNS lookup fails.

Points for discussion:

• Internet access does not guarantee access to the data located at base_url. However, I don't recommend pinging the cluster every time someone runs a test.
• I've set the address to 8.8.8.8 as this is Google's public DNS, instead of http://google.com. It may be better to connect to example.com as this is managed by ICANN, but they don't recommend using this address operationally.
• This function is run inside the test suite's init file. How should I add a test? Or can this be left uncovered?

• Tests added/passed
• Fully documented
• Entry in whats-new.rst

[fmaussion]

Fine by me! This doesn't need tested.

@TimoRoth any opinion on this?