Fix -m flag for IPv6 addresses

Summary:

--my-hostname (or -m) flag is used to pass hostnames test clients are
supposed to talk to. Those are the addresses the tested chef client will
be pulling changed cookbooks from. Unfortunately, it doesn't work if you
try to pass IPv6 address - you'll get an error. In particular,
taste-tester test <test_host> -m [IPv6] won't work.

The underlying problem is that open opening http(s) connection to the
provided hostname we use `uri.host` - this version won't remove [] for IPv6
addresses. There is a replacement `uri.hostname` which will remove
square brackets and make the it work.

Test Plan:

Tested on a host without a change - the error is

```
Failed to open TCP connection to [IPv6]:port (getaddrinfo: Name or service not known)
```

When changed client.rb from `uri.host` to `uri.hostname` everything
work.

Reviewers:

Subscribers:

Tasks:

Tags:

lib/taste_tester/host.rb

@@ -367,7 +367,7 @@ def config
           FileUtils.touch(File.join(taste_tester_dest, 'chefignore'))
           uri = URI.parse('#{url}/file_store/tt.tgz')
           Net::HTTP.start(
-            uri.host,
+            uri.hostname,
             uri.port,
             :use_ssl => #{TasteTester::Config.use_ssl},
             # we expect self signed certificates