Helper for checking env vars in integration tests

De-dupe the checking of environment variables in integration tests to a
helper. Because they all do and require the same thing.
Helper methods for locating integration fixtures

Because I'm about to refactor `#generate_input_config_file` within each of
the integration tests which currently does `File.expand_path("data/…`.
Depend on vcloud-core ~> 0.1.0

For the new templating support added in alphagov/vcloud-core#35
Expose ConfigLoader vars_file on #update

So that we can make use of templated configurations. This will first be
consumed by the integration tests which currently use ERB. Then it will be
exposed to the binstub.
Use mustache templating for EGS integration tests

Use the new functionality in Vcloud::Core::ConfigLoader which renders config
files from Mustache templates and YAML variable files rather than this
project's own ERB helper.

Not doing any other refactoring yet.
Use mustache templating for FW integration tests

Use the new functionality in Vcloud::Core::ConfigLoader which renders config
files from Mustache templates and YAML variable files rather than this
project's own ERB helper.

Not doing any other refactoring yet.
Use mustache templating for LB integration tests

Use the new functionality in Vcloud::Core::ConfigLoader which renders config
files from Mustache templates and YAML variable files rather than this
project's own ERB helper.

Not doing any other refactoring yet.
Use mustache templating for NAT integration tests

Use the new functionality in Vcloud::Core::ConfigLoader which renders config
files from Mustache templates and YAML variable files rather than this
project's own ERB helper.

Not doing any other refactoring yet.
Remove ErbHelper

This is no longer used now that the tests have been converted to use
Mustache templating from Vcloud::Core::ConfigLoader.
Correct name of CLI argument

It should ideally match the name of the block above. It's kind of annoying that
Methadone doesn't care about this. Changes this:
```
➜  vcloud-edge_gateway git:(1819f2c) b vcloud-configure-edge
parse error: 'resource' is required

Usage: vcloud-configure-edge [options] resource
```

To this:
```
➜  vcloud-edge_gateway git:(64815072-mustache_config_templating) b vcloud-configure-edge
parse error: 'config_file' is required

Usage: vcloud-configure-edge [options] config_file
```
Expose mustache templating from CLI

Add an optional second argument that allows you to pass a vars file and
triggers template rendering in Vcloud::Core::ConfigLoader.
Merge branch 'master' into 64815072-mustache_config_templating

Conflicts:
	vcloud-edge_gateway.gemspec
Add templating to CHANGELOG.
Merge pull request #52 from alphagov/64815072-mustache_config_templating

[#64815072] Use mustache templating from vcloud-core