Add back SimpleCov

This partially reverts d3ae38c88841fd1f29e5edc90b4c9f6738234a7b with the
following differences:

Configure the SimpleCov behaviour so that it doesn't report coverage
when `rake` completes and acts only as a 'red light' if coverage drops
significantly (by 20%) or drops below a percentage (80%).

Note that to silence the 'Coverage report generated for' message that
reports the coverage percentage when SimpleCov completes, we
monkey-patch the SimpleCov::Formatter::HTMLFormatter class.

Also, `SimpleCov#result.format!` is called by default when SimpleCov
exits[1], so there's no need for us to explicitly call it.

The SimpleCov::Formatter::HTMLFormatter#output_message _only_ prints the
'Coverage report generated for' message, so by monkey-patching it we
don't lose the other warnings that we want to keep (mentioned above).
Pin SimpleCov to version 0.7.1

Pin SimpleCov to version 0.7.1 to avoid the bug in SimpleCov 0.8.x
affecting exit codes:

https://github.com/colszowka/simplecov/issues/281

Note that I had to change `profiles` to `adapters`, as the naming
changed in version 0.8.0[1].

[1]:
https://github.com/colszowka/simplecov/commit/60f4717c39dd6919d18f263c689f7d75738d6ce8#diff-4ac32a78649ca5bdd8e0ba38b7006a1eR16
Adjust SimpleCov thresholds

Set the minimum coverage to 100% to match the current coverage, as
discussed with @annashipman.

Also, remove the maximum coverage drop as this will never be triggered
given the minimum coverage we have set.
Merge pull request #32 from gds-operations/72169016-simple-cov-red-light

[#72169016] Add back SimpleCov
Update READMEs with new integration test details
Link to contributing guidelines

...instead of duplicating some of that info in the README. Also remove some references that weren't used.
Rename the integration test that tests net_launch

As per the standard in the other gems.
The README was wrong about required parameters

These tests only require two parameters to be set.

Add the template config yaml file, add the file itself to gitignore and update the integration tests README to indicate what the required parameters are.
Use vCloud Tools tester instead of env vars
Update Jenkins files so CI works

This obtains the configuration from GitHub Enterprise. It also cleans up before and after itself.