Add env var section to topology web page

Add description of SLURM_TOPOLOGY_ADDR and SLURM_TOPOLOGY_ADDR_PATTERN
environment variables to the topology web page.
sview - Fix regression where the Node tab wasn't able to
add/remove columns.

caused by commit 68f0f5db21d2f46f9219c72880149655d345a090
Merge remote-tracking branch 'origin/slurm-2.6'