Improve verify_and_configure()

[sajith]

Like #302, this is another follow-up to the discussion on #301, and this might be a matter of opinion: IMO verify_and_configure() would be better as two explicit steps: a generate_configuration() method to generate configuration (hostnames, sliver/bastion ssh keys, bastion username, project ID), and then a verify_configuration() method to validate the configuration.

• This way the steps will be explicit rather than implicit.
• People who want to manually configure things or temporarily override configuration using environment variables can benefit from validate_configuration(). (Speaking of, I'm not sure we give environment variables the "correct" priority, and that should be a separate issue.)

It would be also useful if validate_configuration() can return a list of errors, rather than return at the first failure.