Skip ovs_ and ebtables_dump if needed commands not present
Skip ovs_ and ebtables_dump if needed commands not present

Skip with a notification that a command was not found when trying
to run a dump that relies on optional external command.

Otherwise we produce noise in the error output that is misleading.

Change-Id: I0e3d99da8c54d2912463ecba7c1783864a7c7fc7
Closes-Bug: #1548833
Closes-Bug: #1506873

bigswitch/devstack,dirkmueller/devstack,richliu/devstack,richliu/devstack,samgoon/devstack,bswartz/devstack,openstack-dev/devstack,noironetworks/devstack,samgoon/devstack,dtroyer/devstack,openstack-dev/devstack,nawawi/openstack,noironetworks/devstack,Mahito/devstack,Mahito/devstack,bswartz/devstack,dirkmueller/devstack,dtroyer/devstack,group-policy/devstack,bigswitch/devstack,group-policy/devstack,nawawi/openstack