Refactored host attributes to plugin registry
* Legacy plugin registration works for the common use cases.
Using declare_host_attribute() works as in previous versions.
The attribute classes have been replaced by factories that
create classes for the attributes on demand. This works for
the commonly used attribute types like ValueSpecAttribute,
NagiosTextAttribute and so on.
* The attribute declarations that used subclassing have already
been refactored. The others will be refactored in a future
commit.
* The host attributes registration and access to host attributes
is now exclusively done via host_attribute_registry.
CMK-1588
Change-Id: Ie7ff326797f0cf3685ca8102ce2263e3d1863840