optimize aug_match() query for all ifcfg files related to an interface
    
    This resolves:
    
     https://bugzilla.redhat.com/show_bug.cgi?id=1269613
    
    The original augeas search term used by netcf to find, for example, all the
    ifcfg files associated with device "br1" was:
    
         "/files/etc/sysconfig/network-scripts/*[ "
         "DEVICE = 'br1' or BRIDGE = 'br1' or MASTER = 'br1' or MASTER = "
         "../*[BRIDGE = 'br1']/DEVICE ]/DEVICE"
    
    This is *extremely* inefficient - on a test host with 514 host
    bridges, each with an attached vlan interface, a dumpxml of all
    toplevel interfaces took 6m40s (*after* installing an augeas that
    included augeas upstream commits a659f09a, 41e989ca, and 23d5e480
    which were all pushed after the augeas-1.4.0 release).
    
    In these two messages:
    
     https://www.redhat.com/archives/augeas-devel/2015-October/msg00003.html
     https://www.redhat.com/archives/augeas-devel/2015-October/msg00004.html
    
    David Lutterkort suggested changing the search term to:
    
      "(/files/etc/sysconfig/network-scripts/*[(DEVICE|BRIDGE|MASTER) = 'br1']"
      "|/files/etc/sysconfig/network-scripts/*[MASTER]"
      "[MASTER = ../*[BRIDGE = 'br1']/DEVICE ])/DEVICE
    
    That's what this patch does. Testing shows that it is functionally
    equivalent, and reduces the dumpxml time in the previously described
    test from 6m40s down to 17 seconds.