Clarify that namespaces are not deleted by default for DHCP and L3 agents

All namespaces for DHCP and L3 agent in Neutron are not deleted
by default, this note indicates that and also clarifies that this
behaviour can be changed for both agents via configuration files

Change-Id: If9b7431fb29ea52552147c92847da465801bb0f1
Closes-bug: #1402739
Related-bug: #1052535
Merge "Clarify that namespaces are not deleted by default for DHCP and L3 agents"