Change format of the empty status message
Change format of the empty status message

Having 'null' instead of an empty string for an empty status message
increases the complexity of the code with no real improvement.

This patch modifies the monitoring agent design document solving this issue.

Signed-off-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>
Reviewed-by: Bernardo Dal Seno <dcdd79c5479cb9d1329ec01d126f02cb045aa103@google.com>

yiannist/ganeti,yiannist/ganeti,ganeti/ganeti,leshchevds/ganeti,andir/ganeti,onponomarev/ganeti,mbakke/ganeti,apyrgio/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,andir/ganeti,onponomarev/ganeti,dimara/ganeti,ganeti/ganeti,leshchevds/ganeti,andir/ganeti,mbakke/ganeti,bitemyapp/ganeti,mbakke/ganeti,ganeti/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti,apyrgio/ganeti,grnet/snf-ganeti,yiannist/ganeti,dimara/ganeti