templatize default configs

[choey]

For example, the following are hard-coded:

• gremlin port
• storage backend / host
• db cache size
• timeouts...

Even though configs are overridable by creating a sub-image or mounting /conf/, it would be convenient to templatize some of these configs so convenience.

Like #1, templatized configs (which should have defaults in case not specified) should be settable via environment variables.