ceph-common: purge ceph.conf file

ceph-common: purge ceph.conf file

Since ##461 we have been having the ability to override ceph default
options. Previously we had to add a new line in the template and then
another variable as well. Doing a PR for one option was such a pain. As
a result, we now have tons of options that we need to maintain across
all the ceph version, yet another painful thing to do.
This commit removes all the ceph options so they are handled by ceph
directly. If you want to add a new option, feel free to to use the
`ceph_conf_overrides` variable of your `group_vars/all`.

Risks, for those who have been managing their ceph using ceph-ansible
this is not a trivial change as it will trigger a change in your
`ceph.conf` and then restart all your ceph services. Moreover if you did
some specific tweaks as well, prior to run ansible you should update the
`ceph_conf_overrides` variable to reflect your previous changes.

To avoid service restart, you need to know a bit of ansible for this,
but generally the idea would be to run ansible on a dummy host to
generate the ceph.conf, then scp this file to all your ceph hosts and
you should be good.

Closes: #693
Signed-off-by: Sébastien Han <seb@redhat.com>