debian/ceph-common.postinst: set user directory using adduser

debian/ceph-common.postinst: set user directory using adduser

now that adduser allows us to set its home directory, we can do
this using adduser instead of using usermod. this change also
silences the warning from lintian
"maintainer-script-lacks-home-in-adduser". lintian complains if
`adduser --system` is called without passing `--home` option.

also, take this opportunity to s/-c/--comment/ in the command line
of `usermod`, for better readability.

Fixes: https://tracker.ceph.com/issues/64069
Signed-off-by: Kefu Chai <tchaikov@gmail.com>