debian: specify interpreters for ceph-mon and ceph-osd postinsts

debian: specify interpreters for ceph-mon and ceph-osd postinsts

These were previously missing. The requirement for interpreters is in
Debian policy section 10.4:
https://www.debian.org/doc/debian-policy/ch-files.html#s-scripts

Debian's packaging already adds the #! to these two postinsts. In
practice, a text executible without a #! line will likely be executed
by the calling shell, so a lot of the time we'd get away with it
unless the administrator is using an incompatible shell like tcsh.

This behaviour of shells is documented in POSIX section 1(e)(i)(b)
here:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01_01

Signed-off-by: Matthew Vernon <mvernon@wikimedia.org>