Connections and clients for hosts no longer in host list not cleaned up when host list is re-assigned on an existing client

[pkittenis]

Bug reports

Steps to reproduce:

client = ParallelSSHClient(['host1', 'host2'])
output = client.run_command('echo me')
client.join(output)
client.hosts = ['host2', 'host3']
output = client.run_command('echo me')
client.join(output)

assert (0, 'host1') not in client._host_clients

Returns False.

Client for host1 is not disconnected, dictionary that holds clients keeps increasing in size as host list is re-assigned.

Expected behaviour: Clients no longer in host list are cleaned up.

Actual behaviour: Connections remain active, memory use of parallel client increases.

Additional info: 1.13.0