Expand home dir for user rbenv_path to support quoted ENV in SSHKit

[cshaffer]

As a result of capistrano/sshkit#250 for user installs of rbenv result in incorrect ENV in the mapped bins.

So what used to be this:
$ RBENV_ROOT=~/.rbenv RBENV_VERSION=2.2.2 ~/.rbenv/bin/rbenv exec gem query --quiet --installed --name-matches ^bundler$

Became this:
RBENV_ROOT="~/.rbenv" RBENV_VERSION="2.2.2" ~/.rbenv/bin/rbenv exec gem query --quiet --installed --name-matches ^bundler$

In other words path expansion was no longer occurring due to the quotes added by SSHKit. This is easy to workaround by overriding the default rbenv_path property, however this PR sets the default to the expanded path so that is no longer necessary.

[cshaffer]

Whoops looks like deploy_user isn't valid anymore: capistrano/capistrano#1399. Closing until I come up with a better way.