doc: add journalctl example for grepping the logs
doc: add journalctl example for grepping the logs

I had to dig around for awhile to figure this out, so
this adds an example on how to grep journalctl nova logs
for a server instance UUID.

Change-Id: I6a5c47fbcba3af1822e2f9efc2ac20ebe0387f3f

dtroyer/devstack,group-policy/devstack,nawawi/openstack,noironetworks/devstack,dtroyer/devstack,group-policy/devstack,nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,noironetworks/devstack