Provide standard interface on RemoteAccount for getting a hostname that can be used from the test driver

[ewencp]

Currently VagrantCluster sets up externally_routable_ip, but this field isn't included for other cluster types and its naming is very unclear. We should standardize what needs to be provided, and RemoteAccount can help out by handling the common case where hostname and the hostname used by the driver machine are the same.

Note that ssh_hostname isn't sufficient because on virtualbox vagrant clusters that value is always 127.0.0.1. ssh_hostname also isn't ideal for AWS clusters because it uses the public IP, but generally the driver is running in AWS as well and can use the AWS-internal IP that doesn't cost money for bandwidth.

[ewencp]

Maybe also document what all the fields in RemoteAccount mean because having hostname, ssh_hostname, and externally_routable_ip will be very confusing for new users.