testing: use juju-mongodb when appropriate

testing: use juju-mongodb when appropriate

This should address parts of both bug #1301353 and bug #1304770.
Instead of requiring a 'mongod' to be in PATH, we'll also search for
/usr/lib/juju/bin/mongod which is from the juju-mongodb package.
For core tests, we prefer the juju-mongodb version (since jujud itself
will prefer it), and for store tests we prefer 'mongod' on PATH since
it will have the v8 engine and allow us to run all the tests.

This also auto-detects when noTestMongoJS should be set because it is
using the /usr/lib/juju/bin/mongod executable. Note that this
autodetection *fails* for people who have been hacking around support by
symlinking /usr/lib/juju/bin/mongod back into their PATH. So I didn't
remove the ENV variable, but it should no longer be necessary.

I'm targetting 1.18 because with this patch I think Juju CI will have to
do less hackery to have the test suite JustWork for them.

https://code.launchpad.net/~jameinel/juju-core/1.18-trusty-mongodb-1301353/+merge/218928

(do not edit description out of merge proposal)

[jameinel, 2014-05-09 07:51:40]

Please take a look.

[fwereade, 2014-05-09 08:01:25]

LGTM, just trivials/suggestions

https://codereview.appspot.com/97240043/diff/1/testing/mgo.go
File testing/mgo.go (right):

https://codereview.appspot.com/97240043/diff/1/testing/mgo.go#newcode148
testing/mgo.go:148: // is available, it will be use preferentially
s/use/used/

https://codereview.appspot.com/97240043/diff/1/testing/mgo.go#newcode160
testing/mgo.go:160: }
maybe log mongopath here and not above?

[jameinel, 2014-05-09 08:08:57]

Please take a look.

https://codereview.appspot.com/97240043/diff/1/testing/mgo.go
File testing/mgo.go (right):

https://codereview.appspot.com/97240043/diff/1/testing/mgo.go#newcode148
testing/mgo.go:148: // is available, it will be use preferentially
On 2014/05/09 08:01:25, fwereade wrote:
> s/use/used/

Done.

https://codereview.appspot.com/97240043/diff/1/testing/mgo.go#newcode160
testing/mgo.go:160: }
On 2014/05/09 08:01:25, fwereade wrote:
> maybe log mongopath here and not above?

Done.