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 below deps which added python3.8 support
Markupsafe==1.1.1
cffi==1.14.0
greenlet==0.4.15
PyYAML==3.13

Story: #2007865
Task: #40200

Closes-Bug: #1886298

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

Change-Id: I9857dc20057b97a2d0bae5b0b3410fa958bdc74f

bindep.txt

@@ -10,7 +10,7 @@ libdbus-1-dev [platform:dpkg]
 libdbus-glib-1-dev [platform:dpkg]
 libffi-dev [platform:dpkg]
 libffi-devel [platform:rpm]
-libssl-dev [platform:ubuntu-xenial]
+libssl-dev [platform:ubuntu]
 libuuid-devel [platform:rpm]
 locales [platform:debian]
 python-dev [platform:dpkg]

lower-constraints.txt

@@ -4,7 +4,7 @@ asn1crypto==0.23.0
 Babel==2.3.4
 bandit==1.4.0
 cachetools==2.0.0
-cffi==1.7.0
+cffi==1.14.0
 cliff==2.8.0
 cmd2==0.8.0
 contextlib2==0.4.0
@@ -24,7 +24,7 @@ future==0.16.0
 futurist==1.2.0
 gitdb==0.6.4
 GitPython==1.0.1
-greenlet==0.4.10
+greenlet==0.4.15
 hacking==1.1.0
 idna==2.6
 iso8601==0.1.11
@@ -36,7 +36,7 @@ jsonschema==2.6.0
 keystoneauth1==3.5.0
 kombu==4.0.0
 linecache2==1.0.0
-MarkupSafe==1.0
+MarkupSafe==1.1.1
 mccabe==0.2.1
 monotonic==0.6
 msgpack-python==0.4.0
@@ -82,7 +82,7 @@ python-mimeparse==1.6.0
 python-neutronclient==6.7.0
 python-subunit==1.0.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 repoze.lru==0.7
 requests==2.14.2
 requests-mock==1.2.0