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.

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.