bootloader: Extend check-config to log non-standard properties

Addresses the issue that there is no easy way to find mistyped
property names in dCache configurations. 'dcache check-config'
now logs messages at INFO level about property declarations that
are not known to dCache.

Messages are only logged at INFO as dCache allows the user to
add custom properties used to declare other properties, or
used by third party scripts. Such properties are thus not
errors.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6111/