core: Add ExternalVariable entity

1) Creates external_variable table
2) Adds stored procedures to access external_variable table
3) Adds DAO to access the entity

This table will be used to share fence_kdump listener heartbeat between
engine and fence_kdump listener.

Change-Id: I61ffd646c102c01c34478309d072f74a8f7d7edf
Bug-Url: https://bugzilla.redhat.com/1079821
Signed-off-by: Martin Perina <mperina@redhat.com>