For `waitForTerminal` check if the build timeout is actually shorter
then the terminal session timeout. If so, parse the time left, show it
to the user and use it as a timeout as well.

The reason `timeout` is being updated and used for `time.After` (in the
select statement) is so we have a good seperation between when
`ctx.Done` is being called because the user actually cancelled the build
and when the context deadline is reached.

Add `RoundDuration` which is the same as `time.duration.Round` but since
we are using go1.8 it is missing.

Add a test cases for each scenario `waitForTerminal` has.