Add tilde support for leafnode credentials

[variadico]

Currently, the leafnode configuration doesn't expand the ~ character to the user's home directory. This change allows users to use ~ in the configuration file when declaring the path to their credentials.

leafnodes {
  remotes [
    {
      url: nats-leaf://127.0.0.1:2222
      account: foobar
      credentials: ~/my.creds
    }
  ]
}

Signed-off-by: Jaime Piña jaime@synadia.com
/cc @nats-io/core

[kozlovic]

LGTM

[derekcollison]

LGTM - Thanks!