* Introduce a new package under ciutil for individual CI systems. Split-out each CI system with env var detection for each.
* Add Bitbucket Piplines env var detection.
* Update changelog with note about adding Bitbucket Pipelines detection.
* Rename the CI system structs.
* Move files from ciutil/systems to ciutil. Un-export some types that don't need visibility beyond the ciutil package.
* Un-export DetectSystem function and the System type.
* Add a test for CI systems which we only know by name and nothing else, i.e. those with just a baseCI implementation.
* Add a var for PRNumber. Add an environment metadata key for PR number.
* Move the detection of PULUMI_CI_SYSTEM into vars.DetectVars(). Set the PRNumber CI property based on respective env vars from each CI system.
* Add Azure Pipelines build variables.
* Add tests for DetectVars.
* Added changelog entry for Azure Pipelines.
* Capture the value of env var being modified for the ciutil unit test, and restore their values at the end of them.
* Simplify the DetectVars function by moving the Pulumi CI system code into the switch-case expression.
* Rename the Pulumi CI system to Generic CI. Include the GenericCI system in the test case for DetectVars.
This change adds GitLab CI support, by sniffing out the right
variables (equivalent to what we already do for Travis).
I've also restructured the code to share more logic with our
existing CI detection code, now moved to the pkg/util/ciutil
package, and will be fleshing this out more in the days to come.
