[goal] Migrate testing to ubuntu focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Fixing:
- bug#1886298
Bump the lower constraints for required deps which added python3.8 support
in their later version.

Story: #2007865
Task: #40225

Closes-Bug: #1886298

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal>

Change-Id: Iffd84ecdcb9a44137b0d51b258183c80ab6f030a

lower-constraints.txt

@@ -2,11 +2,11 @@ alabaster==0.7.10
 appdirs==1.3.0
 asn1crypto==0.23.0
 Babel==2.3.4
-cffi==1.7.0
+cffi==1.14.0
 cliff==2.8.0
 cmd2==0.8.0
 coverage==4.0
-cryptography==2.1
+cryptography==2.7
 debtcollector==1.2.0
 decorator==3.4.0
 deprecation==1.0
@@ -61,7 +61,7 @@ python-openstackclient==3.12.0
 python-subunit==1.0.0
 python-swiftclient==3.2.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 requests-mock==1.2.0
 requests==2.14.2
 requestsexceptions==1.2.0