(PUP-3258) node exec indirector: default to environment, not its name

Nodes can be classified through a local executable that prints YAML with
a node specification. This is enabled through the node_terminus=exec option.
Since PUP-2519, this setting would break `puppet apply`. Trying to apply
any manifest would lead to Puppet applying the default environment's site
manifest instead.

The reason for this is that the node terminus will include the current
environment's name in the node lookup result. This in turn makes Puppet
look up the environment per name. This is not the desired behavior, because
Application::Apply performs the lookup and further operations with a localized
environment that overrides the site manifest.

Solved by including the current environment object in the returned node object
instead of its name.
(maint) reduce repitition in Application::Apply integration spec
(maint) Remove duplicate file handling code

The tests had their own way of creating files and directories for
testing. All of these different ways of creating files and directories
already existed in PuppetSpec::Files. This unifies them.
Merge branch 'pr/3144' into stable

* pr/3144:
  (maint) Remove duplicate file handling code
  (maint) reduce repitition in Application::Apply integration spec
  (PUP-3258) node exec indirector: default to environment, not its name

Closes GH-3144