Add missing key_server_hostname variable (#1449)

Allow key_server_hostname to be specified when initializing a PoolManager to allow custom SNI to be overridden.

src/urllib3/poolmanager.py

@@ -47,6 +47,7 @@
     'key__socks_options',  # dict
     'key_assert_hostname',  # bool or string
     'key_assert_fingerprint',  # str
+    'key_server_hostname', #str
 )
 
 #: The namedtuple class used to construct keys for the connection pool.