Avoid failure on transient requirements in CI image (apache#8892)

When you build from the scratch and some transient requirements
fail, the initial step of installation might fail.

We are now using latest valid constraints from the DEFAULT_BRANCH
branch to avoid it.

Prepare release candidate for backport packages (apache#8891)

After preparing the 2020.5.19 release candidate and
reviewing the packages, some changes turned out to be necessary.

Therefore the date was changed to 2020.5.20 with the folowing
fixes:

* cncf.kubernetes.example_dags were hard-coded and added for all
  packagesa and they were removed
* Version suffix is only used to rename the binary packages not for
  the version itself
* Release process description is updated with the release process
* Package version is consistent - leading 0s are skipped in month
  and day

Update Changelog for 1.10.10rc5

Update Changelog for 1.10.10rc5

Airflow 1.10.10rc4

Airflow 1.10.10rc3

Update Changelog for 1.10.10rc2

Update 1.10.10 Changelog

[AIRFLOW-XXXX] Bump version to 1.10.9 & add Changelog

[AIRFLOW-XXXX] Bump version to 1.10.9 & add Changelog