Bash escaping

We weren't properly escaping the environment variables written in `deploy.env`, which caused issues for some projects.

This PR:
 - Migrates SingularityExecutor from mustache to handlebars templating
 - Introduces a `bashEscaped` handlebars helper, which attempts to properly escape a string (adapted from StringEscapes in https://github.com/brooklyncentral/brooklyn)
 - Updates the `deploy.env` template to use the `bashEscaped` helper

(since handlebars is a superset of mustache, we don't need to update the other templates)

@wsorenson @gchomatas