In CentOS, we expect to have python3 package in 8.x while we expect to
have python2 in 7.x .
Fix unit tests to expect the correct version according to os major
version.
Incorrect version handling about libreswan/openswan package is also
fixed so that we have correct one in CentOS8.
In addition, this patch updates test scope in tempest execution in
tripleo job to avoid timeout error caused by many long-running tests.
Closes-Bug: #1870257
Change-Id: I72302b58161edccb15ca008e11c670caaf94edaf
f7bc1dc4f2