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>
(cherry picked from commit a1a857698cad4f2feeea0b56fd781350e44078b7)