qa/suites: add librados2 to "extra_packages" for upgrade tests

qa/suites: add librados2 to "extra_packages" for upgrade tests

we use the playbook of "testnodes.yml" defined by ceph-cm-ansible for
initializing test nodes, and the role of "testnode" is used by
testnodes.yml. "testnode" requires "qemu-system-x86" or "qemu-kvm"
package to be installed. the qemu in turn depends on librbd1 and
librados2.

before librados3 was introduced, this worked perfectly. because in ceph
repo, qa/packages/packages.yaml defines the default set of packages the
"install" tasks should install. and in that yaml file, librados2 was
listed. so the package management system will overwrite the librados2
installed by ansible playbook with the version specified by the
"install" task, as apt/yum thinks this is what user requires explicitly,
so it's fine to install a different version of librados2.

after librados3 was introduced, librados2 was removed from
qa/packages/packages.yaml. because, by default, we need to install
librados3 instead of librados2 for ready a nautilus cluster. but the
problem is, the packge list also applies to "install" tasks installing
releases before nautilus, where we still need to replace the librados2
installed by ansible.

so, to address this issue, "librados2" is added to "extra_packages" of
the "install" tasks of tests installing old releases to install
librados2 explicitly instead of as a dependency of other ceph packages
like librbd1.

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