GCE provider config should support extracting auth info from JSON file

[Eric Snow]

The GCE console gives you a JSON file containing the auth info you need for the provider (to put in environments.yaml). Andrew suggested that we support extracting the info from the JSON file, which will help them avoid issues that come from copying-and-pasting formatting-sensitive data like that.

[Eric Snow (ericsnowcurrently)]

This is consistently failing on vivid and probably on other platforms. See http://data.vapour.ws/juju-ci/products/version-2483/run-unit-tests-vivid-amd64/build-297/consoleText.

----------------------------------------------------------------------
FAIL: poollist_test.go:114: PoolListSuite.TestPoolListTabularSorted

poollist_test.go:125:
    s.assertValidList(
        c,
        []string{"--name", "myaw", "--name", "xyz", "--name", "abc",
            "--format", "tabular"},
        `
    NAME PROVIDER ATTRS
    abc testType one=true two=well three=maybe
    myaw testType one=true two=well three=maybe
    xyz testType one=true two=well three=maybe

    `[1:])
poollist_test.go:133:
    c.Assert(obtained, gc.Equals, expected)
... obtained string = "" +
... "NAME PROVIDER ATTRS\n" +
... "abc testType three=maybe one=true two=well\n" +
... "myaw testType one=true two=well three=maybe\n" +
... "xyz testType one=true two=well three=maybe\n" +
... "\n"
... expected string = "" +
... "NAME PROVIDER ATTRS\n" +
... "abc testType one=true two=well three=maybe\n" +
... "myaw testType one=true two=well three=maybe\n" +
... "xyz testType one=true two=well three=maybe\n" +
... "\n"

OOPS: 23 passed, 1 FAILED
--- FAIL: TestAll (0.07 seconds)
FAIL
FAIL github.com/juju/juju/cmd/juju/storage 0.388s

[Eric Snow (ericsnowcurrently)]

Oops, wrong bug.

[Eric Snow (ericsnowcurrently)]

http://reviews.vapour.ws/r/1278/