Ceph

we install different versions of precompiled ceph-libboost packages
for different branches when building and testing them on ubuntu test
nodes. for instance,

nautilus, octopus: v1.72
    pacific: v1.73

they share the same set of test nodes. and these ceph-libboost packages
conflict with each other, because they install files to the same places.

in order to avoid the confliction, we should uninstall existing packages
before installing a different version of ceph-libboost packages.