cephconfig: change code style inside config writer

cephconfig: change code style inside config writer

This commit changes the code style of subroutine `write_ceph_config`
to match our style guide [0] more.

Furthermore, the repeated calls to the inner subroutine are replaced
with a loop, while the regular expressions used by the inner `sub` are
now quoted with `qr` to prevent any accidental mis-quotings in the
future.

[0]: https://pve.proxmox.com/wiki/Perl_Style_Guide

Signed-off-by: Max Carrara <m.carrara@proxmox.com>
Tested-by: Friedrich Weber <f.weber@proxmox.com>