Respawn delay

Sometimes, especially for hardware drivers, it helps to pause before restarting a failed node.

This branch adds a configurable delay to the roslaunch respawn feature. If the delay is left unspecified, it defaults to 0, giving the same behavior as before. The value is configured in the node tag:
<node
     name="myname"
     pkg="flaky_hardware"
     type="unreliable_node"
    respawn="True"
    respawn_delay="5.0"
/>
respawn_delay has units seconds.
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