(2.2) dcache: minimal backport to enable remote logging from 2.2 domains

dcache: alarms

For mixed installations where the head and door nodes may be running later versions but pool nodes are not updated from 2.2, it would still be nice for a potential alarm server to receive error or warn events from the pools.

This patch provides a minimal backport of alarm sending (actually, log event sending) capabilities.

  - IAlarms interface for certain constants that are used
  - An appender implementation which adds some properties to the MDC before sending the event
  - an alarms.properties default with only the properties used to configure the appender
  - the necessary xml for the remote appender in logback.xml

Note that this preserves the 2.6 semantics (e.g., the error/warn limitation and the property naming), not the more generalized configuration added to 2.7+.

Target: 2.2
Patch: http://rb.dcache.org/r/5564
Require-notes: yes
Require-book: yes
Acked-by: Tigran

RELEASE NOTES:
BOOK:

Version 2.2 does not contain the full alarm service capabilities (server, store and webpage); however, this patch allows all 2.2 domains to communicate with a potential alarm server by enabling the sending of log events (at the ERROR or WARN levels) to a remote server.  The following properties determine the server endpoint and the level of log events sent:

(one-of?off|error|warn)alarms.remote-logging.level=off

alarms.server.host=localhost

alarms.server.port=60001

The default for 2.2 has been set to "off", unlike later versions, where it is set to warn.
Merge pull request #49 from alrossi/feature/2.2/backport_remote_logging

(2.2) dcache: minimal backport to enable remote logging from 2.2 domains