[Varnish] #735: Support backend control via CLI

[Varnish]

#735: Support backend control via CLI
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 Reporter:  tgr          |       Owner:  phk  
     Type:  enhancement  |      Status:  new  
 Priority:  normal       |   Milestone:       
Component:  varnishd     |     Version:  trunk
 Severity:  normal       |    Keywords:       
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 The attached patches (for trunk and for 2.1, the only difference is Sphinx
 docs) are a PoC of CLI-based backend control for round-robin and random
 directors. It's a quick hack and the code may not be where you want it,
 but it works.

 Rationale:

 - it may be desirable to allow application administrators to
 disable/enable backends without allowing VCL modification,

 - there is presumably no need to disable a backend that is not part of a
 director,

 - health probes should continue when a backend is disabled - it is easy to
 make a decision to disable/enable a backend based on its health status,
 when presented in a format like:

 director1 backend1 Enabled Healthy[[BR]]
 director1 backend2 Disabled Healthy[[BR]]
 director1 backend3 Disabled Sick[[BR]]

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Ticket URL: <http://varnish-cache.org/ticket/735>
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