rest: Simple shortcuts for redis set and get

The class in which this mixin is included to must implemented a
instance_key method which uniquely identifies the object
rest: Better error message if search is tried before proper connection
rest: Add LdapModel#by_attr(key, val)

And complementary by_attr!, by_ldap_attr and by_ldap_attr!
rest: add LdapModel#by_dn!(...)
Allow logger swapping in flog.merge(...)
rest: Device get hostname from puavoHostname
rest: Save global fluent logger $rest_flog

Can be swapped during tests without warnings
rest: Log device boot times to fluentd