Fix gate due to switch to focal

In focal we do not have libffi6.
cffi and greenlet versions in lower-contraints are too old.

Change-Id: Iab3634039845adb649c7fd69d1812b405a61433c

bindep.txt

@@ -5,7 +5,6 @@ gcc [compile test]
 libc6-dev [compile test platform:dpkg]
 libffi-devel [platform:rpm]
 libffi-dev [compile test platform:dpkg]
-libffi6 [platform:dpkg]
 libssl-dev [compile test platform:dpkg]
 python3-dev [compile test platform:dpkg]
 python3-devel [compile test platform:rpm]

lower-constraints.txt

@@ -4,7 +4,7 @@ appdirs==1.3.0
 asn1crypto==0.23.0
 bandit==1.1.0
 cachetools==2.0.0
-cffi==1.7.0
+cffi==1.14.0
 cliff==2.8.0
 cmd2==0.8.0
 contextlib2==0.4.0
@@ -28,7 +28,7 @@ futurist==2.1.0
 gitdb==0.6.4
 GitPython==1.0.1
 gnocchiclient==3.3.1
-greenlet==0.4.10
+greenlet==0.4.15
 hacking==2.0.0
 httplib2==0.9.1
 idna==2.6