mon_command: refactor to pass all data to perl

mon_command: refactor to pass all data to perl

By passing back all return values from the Ceph API (RADOS.xs) to Perl
we are more flexible to make more than just the data available further
up the stack. These values are:

* return code
* status message (can contain useful information)
* data

The Ceph API interaction happens in a child process. We need to en- and
decode the returned hash in JSON to pass it between the child and parent
process.

RADOS.pm::mon_command now returns not just the data, but all information
as a hash ref.  Therefore dependent packages (pve-manager, pve-storage)
need to adapt.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>