Consider ins with same env for scale only

After the introduction of the new env dimension it turned out that the
scale behaviour wasn't adjusted completely. Leading to confusion and
harmful results as reported in soundcloud/bazooka/issues/86. In order
to provide consistent behaviour the `Scale` method exclusively takes
instances with a matching `env` into consideration for ticket
creation.

For `app-proc@rev` tuples with a great amount of existing instances
this will introduce a performance regression as we move from a single
list dir for the `proc-rev` bucket to a retrieval of all instances
with their extended sub-tree. There is room to optimise this
bottleneck by parallelising the retrievals, which is postponed to
keep reasoning about this change-set simple.
Merge pull request #106 from soundcloud/feature/scale-with-env

Only consider instances with same env for scale