Env backend support for keys with underscore

[stela]

This patch allows underscores in paths, even when the "env" backend is used. See #531.

I attempted to add the key-value combinations for all the backend integration tests, but didn't actually run them yet myself besides for the "env" backend. The formats I saw didn't give me reason to suspect underscores aren't handled like other alphanumeric values in other backends, hope that's correct.

[nvanheuverzwijn]

This is a better solution than this #541

Nice work

[Danno040]

So, there's still a "gotcha" with this solution when using prefix for loading a group of keys.

For example, this will fail:

basic.toml

[template]
mode = "0644"
src = "basic.conf.tmpl"
dest = "/tmp/confd-basic-test.conf"
keys = [
  "/database",
]

basic.conf.tmpl

host={{getv "/database/host"}}
port={{getv "/database/user_name"}}

Because everything after the prefix gets "cleaned", the result of GetValues will look like:

map[string]string{"/database/user/name":"something", "/database/host":"host"}

Maybe a minor problem, but worth pointing out.