Validate log level in dbus_prop_set

Currently, setting log level of a component using dbus interface
succeeds with incorrect log level string. The component's actual log
level is set to -1 in this case. Dbus commands that try to retrieve log
levels later fail if a component has incorrect (-1) log level.

This patch fixes dbus_prop_set to fail setting to an invalid log level.
Also fixes an incorrect 'FALSE' symbol in log_mgr.py python module.

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>
Reviewed-by: Jim Lieb <lieb@sea-troll.net>
Acked-by: Frank S. Filz <ffilzlnx@mindspring.com>