pvecm add: require user to navigate out of /etc/pve

pvecm add: require user to navigate out of /etc/pve

If `pvecm add` is issued from /etc/pve (or any subdirectory), it
prints some errors:

    [...]
    shell-init: error retrieving current directory: getcwd: cannot
    access parent directories: Transport endpoint is not connected
    [...]
    successfully added node 'pve-c2' to cluster.
    cannot fetch initial working directory: Transport endpoint is not
    connected at /usr/share/perl5/PVE/CLI/pvecm.pm line 446.

The reason is that `pvecm add` restarts pmxcfs, which re-mounts the
fuse mount at /etc/pve, invalidating pvecm's working directory.

The error messages give the impression that something went wrong.
Indeed, the second error indicates the temporary directory is not
cleaned up. The cluster join itself actually works, though.

The issue could be fixed by chdir'ing to / in `pvecm add`. However,
the user's shell would still remain in the now-invalid /etc/pve,
potentially leading to confusing "transport endpoint not connected"
messages in future interactions.

To avoid this, require the user to chdir out of /etc/pve before
running `pvecm add`.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>