juju test suite should use juju-mongodb

The fix from Jesse was not actually related to *this* bug. This bug is the fact that if you are on Trusty and have "juju-mongodb" installed (say via juju-local), the test suite still fails because it is looking for "mongod" to be in $PATH.
We shouldn't have to add a symlink for the test suite to run (or install mongodb-server). Instead, it should use /usr/lib/juju/bin/mongod if it cannot find mongod in $PATH.

I still want to prefer mongod in $PATH because the charm store tests need the v8 engine that is not available in juju-mongodb.

The fix might be as simple as:
=== modified file 'testing/mgo.go'
--- testing/mgo.go 2014-04-17 00:19:01 +0000
+++ testing/mgo.go 2014-05-08 15:47:55 +0000
@@ -142,7 +142,12 @@
  if inst.Params != nil {
   mgoargs = append(mgoargs, inst.Params...)
  }
- server := exec.Command("mongod", mgoargs...)
+ path, err := exec.LookPath("mongod")
+ if err != nil {
+ // No mongod on path
+ path, err = exec.LookPath("/usr/lib/juju/bin/mongod")
+ }
+ server := exec.Command(path, mgoargs...)
  out, err := server.StdoutPipe()
  if err != nil {
   return err

But I haven't tested it.

The fix from the 1.18 branch accidentally got reverted when landing into trunk, so it needs to be restored.