Fixes LP:#1614330 by supporting more pagers and graceful degradation.

[cmars]

First tries $PAGER, then less, then just writes the terms to
stdout if none of those worked.

(Review request: http://reviews.vapour.ws/r/5484/)

[cmars]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8852

[cmars]

c:/users/admini~1/appdata/local/temp/tmpman5a5/gogo/src/github.com/juju/juju/provider/dummy/environs.go:310:
    c.Assert(err, jc.ErrorIsNil)
... value *errors.Err = &errors.Err{message:"inst.Dial() failed", cause:(*errors.errorString)(0xc082546da0), previous:(*errors.errorString)(0xc082546da0), file:"github.com/juju/testing/mgo.go", line:632} ("inst.Dial() failed: no reachable servers")
... error stack:
    no reachable servers
    github.com/juju/testing/mgo.go:632: inst.Dial() failed

/me rolls again, $$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8854

[cmars]

$$merge$$ plz

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8855

[cmars]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8857

[cmars]

$$merge$$ again. Feels bad to, but tests pass on my development laptop (Go 1.6.3, Ubuntu 16.04, amd64) and all the above failures all seem to be different, and unrelated to this patch.

Are the Juju tests really this flaky in CI? Is this normal?

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju