If the `kerberos` module is available, winrm will attempt to establish a
Kerberized connection to a Windows server. This allows use of Windows
domain accounts, which are quite often the only kinds of accounts
enabled on enterprise networks.

This also pulls the `transport_schemes` variable up into the
`winrm.Connection` class. This lets tests or future modifications alter
the list of available schemas without reaching into `Connection.__init__`.