Update to Centos image 7.4-1711

Add qemu-ev version 2.9.0-16
Changes required for vpp 18.04:
Add selinux policy rpms at required version.
Add mbedtls

JIRA: CSIT-566
JIRA: CSIT-680
JIRA: CSIT-681
JIRA: CSIT-602
JIRA: CSIT-608
JIRA: CSIT-603

Update Centos image version 1.8:
Point to Vault repo specific to for 7.3-1611 for
consistent control of package versions and releases.

Fix for pypcap import problem.

JIRA: CSIT-991

This fix forces the downloads of the complete package for all the pip python
packages to be installed on Centos regardless of the Python metadata on
the host.

Change-Id: Id651dd113eec41db738ac1e10c92d1074e89b1de
Signed-off-by: Thomas F Herbert <therbert@redhat.com>