Convert tests to use vCloud Tools Tester

Convert integration tests to use the [vCloud Tools
Tester](rubygems.org/gems/vcloud-tools-tester) gem, which replaces the
use of environment variables to define test parameters.

Adds a new development dependency for 'vcloud-tools-tester' and a
`vcloud_tools_testing_config.yaml.template` file. Also, update the
README and .gitignore file accordingly.
Don't reset edge gateway after integration tests

Currently, we reset the edge gateway configuration both before and after
the integration tests are run.

Because the edge gateway is reset before the tests are run, it should
not be necessary to reset it afterwards. Furthermore, this extends the
time it takes for the integration tests to complete as evidenced by the
'VCLOUD_NO_RESET_VSE_AFTER' environment variable which this commit
removes.
Move remove_temp_config_files to IntegrationHelper

Move the `remove_temp_config_files` method in the integration tests to
the IntegrationHelper class to reduce duplicated code.
Prefer @test_data to individual instance variables

Remove the individual instances variables used to store test parameters
and use the @test_data hash instead.

Also update each place where these parameters were referred to.
Merge pull request #72 from alphagov/71835572-vcloud-tools-tester

[#71835572] Convert tests to use vCloud Tools Tester