builder/openstack: Support IPv6 access address for Openstack

builder/vmware: fix typo

s/Detatching/Detaching/

builder/openstack: Increase default SSH timeout from 1m -> 5m

Many cloud providers have a minimum charge of 1 hour, and if there are DNS problems we can hit the 1 minute timeout easily.

Waiting five minutes gives more of a margin of error.

We still prefer the IPv4 address, but if there is an IPv6 address and no IPv4 address, use IPv6.

communicator/ssh: If PKCS1 parsing of the SSH key fails, try PKCS8

builder/openstack: If no image is generated, just return nil 

This mirrors what is done in other providers e.g. Amazon providers

This implements more robust interrupt handling by propagating cancels
throughout more core Packer components.