task/install: assert on installed_version > downgrade_version

[tchaikov]

Signed-off-by: Kefu Chai kchai@redhat.com

[tchaikov]

this should fix failures like http://pulpito.ceph.com/kchai-2018-12-31_06:03:07-rados-wip-kefu2-testing-2018-12-31-1006-distro-basic-smithi/3408958/

where we have

2018-12-31T07:52:17.701 INFO:teuthology.task.install.rpm:Downgrading packages: librbd1, librados2
2018-12-31T07:52:17.701 INFO:teuthology.orchestra.run.smithi023:Running:
2018-12-31T07:52:17.701 INFO:teuthology.orchestra.run.smithi023:> rpm -q librbd1 --qf '%{VERSION}-%{RELEASE}'
2018-12-31T07:52:17.752 INFO:teuthology.orchestra.run.smithi129.stdout:10.2.5-4.el7
2018-12-31T07:52:17.753 INFO:teuthology.packaging:The installed version of librbd1 is 10.2.5-4.el7
2018-12-31T07:52:17.754 ERROR:teuthology.contextutil:Saw exception from nested tasks
Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/contextutil.py", line 30, in nested
    vars.append(enter())
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/task/install/__init__.py", line 256, in install
    install_packages(ctx, package_list, config)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/task/install/__init__.py", line 125, in install_packages
    ctx, remote, pkgs[system_type], config)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/parallel.py", line 85, in __exit__
    for result in self:
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/parallel.py", line 99, in next
    resurrect_traceback(result)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/parallel.py", line 22, in capture_traceback
    return func(*args, **kwargs)