Some PLAB dslice manager updates:

- in addition to asking the dslice agent (on plab) for a list of available
  nodes, we now also fping them all to weed out unresponsive ones.  One problem
  here is that several plab nodes block ICMP; could be solved by pinging with
  nmap (tries both a ICMP, and TCP ping).  This affects the plabdaemon getfree
  command, and subsequently which plab nodes appear as "up" in the DB

- Changed slice naming scheme:  we now append the experiment index onto the
  slice name to try to ensure uniqueness (emulab_<pid>_<eid>_<idx>)

- Modified plabnode to try to cope with flaky nodes - there is some retry
  code in there now

- Added the "fixsudo" shell script which is run very first as root (via the
  cumbersome "su" command) to fix sudoers for later sudo use on plab nodes.