.travis.yml: allow 30 minutes of inactivity in cloud tests

[OddBloke]

We're seeing CI failures currently because a LXD image takes more than
10 minutes to download if the transatlantic link to Canonical's DC is
heavily contested.

This uses a helper provided by Travis[0] to avoid cancellation due to
lack of output for 30 minutes, which should be sufficient to download an
image even at the lowest speeds we've been seeing.

[0] https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received

[OddBloke]

So it looks like travis_wait swallows the output of the command it's wrapping, so this is probably only a short-term solution.

[raharper]

Could we modify the cloud_tests default timeout in CI only ?

[OddBloke]

Could we modify the cloud_tests default timeout in CI only ?

We aren't hitting a timeout internal to our code at all. Travis will consider any job that hasn't emitted output for 10 minutes dead, and kill it. All travis_wait does is emit output every few seconds to avoid this check triggering.

[raharper]

I see. Let's see if this is enough.