plugins/hostlogger: Replace dbus call with signal

The latest version of the Hostlogger supports multi-host mode, each host
is served by a separate service instance.
The service instance now uses SIG_USR1 signal to flush collected messages
to a file.

Tested: Run dreport inside the BMC console, the result dump archive
contains host console logs.

Change-Id: I2ba926dad79b362d0f4abac50619d7969a4f28f7
Signed-off-by: Artem Senichev <a.senichev@yadro.com>
Tested-by: Artem Senichev <a.senichev@yadro.com>