* 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.
