Fix failing centos jobs due to yum replacing python34 with python36.

Fix failing centos jobs due to yum replacing python34 with python36.

vppapigen is failing because the ply library is installed under
python34.

09:04:49 Dependencies Resolved
09:04:49
09:04:49 ================================================================================
09:04:49  Package                Arch           Version               Repository    Size
09:04:49 ================================================================================
09:04:49 Installing:
09:04:49  python36               x86_64         3.6.6-5.el7           epel          66 k
09:04:49      replacing  python34.x86_64 3.4.9-2.el7
09:04:49  python36-libs          x86_64         3.6.6-5.el7           epel         8.5 M
09:04:49      replacing  python34-libs.x86_64 3.4.9-2.el7

----
09:07:27 cd /w/workspace/vpp-verify-master-centos7/build-root/build-vpp-native/vpp/vlibmemory && mkdir -p /w/workspace/vpp-verify-master-centos7/build-root/build-vpp-native/vpp/vlibmemory && /w/workspace/vpp-verify-master-centos7/src/tools/vppapigen/vppapigen --includedir /w/workspace/vpp-verify-master-centos7/src --input /w/workspace/vpp-verify-master-centos7/src/vlibmemory/memclnt.api JSON --output /w/workspace/vpp-verify-master-centos7/build-root/build-vpp-native/vpp/vlibmemory/memclnt.api.json
09:07:27 Traceback (most recent call last):
09:07:27   File "/w/workspace/vpp-verify-master-centos7/src/tools/vppapigen/vppapigen", line 4, in <module>
09:07:27     import ply.lex as lex
09:07:27 ModuleNotFoundError: No module named 'ply'

Change-Id: Ica3b408991d26262418576cffc848fb2a8cb0224
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>