Replace whitelist/blacklist terminology with allowlist/denylist

[findkim]

Update consul-template to use inclusive language, notably replacing the usage of whitelist/blacklist.

Consul Template version

consul-template -v
 v0.25.0 ()

Expected behavior

Configuration options for exec

exec {
  env {
    allowlist = []
    denylist = []
  }
}

Actual behavior

exec {
  env {
    whitelist = []
    blacklist = []
  }
}

References

• This mirrors Consul's efforts in Replace whitelist/blacklist terminology with allowlist/denylist consul#7971
• Config documentation in README.md

[eikenb]

Note we'll need to keep backwards compatibility whitelist/blacklist in the code for some time to come. But we can replace them in the docs and help.