Eradicate eventlet and fix bug lp:959221

The bug is simple: whenever swift uploads to a Swift with SSL,
it uses 100% CPU. It happens because we use HTTPSConnection from
eventlet that loops like that, while holding the interpreter lock.

Now, it could be fixed in eventlet, but let's try something more
natural: drop the eventlet's HTTP client. We do not use green
threads in the client anymore, so it's not like we need it for that.

Note that in most cases clients do not use the BufferedHTTPConnection
either, because it's only installed on Swift server nodes, not on
workstations. Get rid of that too.

bug: 959221
Change-Id: I1eb932779d4171598b3efaa043f817b9c6c995c4