AcceptanceIsolatedEnvironment - add attr_accessor for :env to allow setting custom env vars

[jantman]

As I mentioned in #15, I was having serious issues with specs that actually run vagrant add and vagrant up and spin up a VirtualBox VM. It seems that VirtualBox was storing state between spec runs, which caused failures (VBox was dying with errors trying to read a VirtualBox.xml-tmp under a tmpdir from a previous spec).

This just adds an attr_accessor for env in AcceptanceIsolatedEnvironment, which allows setting environment variables after creation. These could be things like API credentials, or in my case (to solve the VirtualBox issue), environment.env['VBOX_USER_HOME'] = environment.homedir.to_s.

Essentially, this allows setting environment variables to values not known until after the IsolatedEnvironment is instantiated, such as values which include or are based on the tempdir path.

[fnordfish]

I had a similar issue when upgrading vagrant-spec for vagrant-dns. Here is what I did:

# vagrant-spec.config.rb
Vagrant::Spec::Acceptance.configure do |c|
  # ...
  c.instance_variable_set(:@env, c.env.merge('VBOX_USER_HOME' => "{{homedir}}"))
end

The Vagrant::Spec::Acceptance::Configuration instance is passed into the AcceptanceIsolatedEnvironment and it's env is merged. However, there is no way of setting it (the nice way), hence the hacky instance_variable_set.

It would nice to convert the attr_reader :env into attr_accessor :env (https://github.com/mitchellh/vagrant-spec/blob/master/lib/vagrant-spec/acceptance/configuration.rb#L20)

[hashicorp-cla]

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