[rancid] minor change to handle nexus 4000s

[Lee]

The Nexus 4000s don't support the 'show environment power' command, so
nxrancid doesn't collect the config.  What works for me is to change
ShowEnvPower to return 1 instead of -1 for /\% Invalid command at /

I also added a 'next if (/^\s*\^\s*$/);' so it doesn't add a
!Env:                                 ^
line in the saved config file

Lee