doc: make instructions to get an updated cephadm common

doc: make instructions to get an updated cephadm common

As discussed in person and over the ceph orch weekly, we want all users
to use a recent supported version of cephadm. Previously, the
instructions only had those downloading cephadm with curl using the
"add-repo" and "install" commands to get a up-to-date cephadm build.
According to ADK we've seen cases of users get "old" distro packages
in the past. Change the instructions so that the "update cephadm" steps
are common after acquiring a "bootstrap copy" of cephadm.

Signed-off-by: John Mulligan <jmulligan@redhat.com>