PLATFORM-664 vary memcache by environment

Use ``gethostname()`` as a prefix for both local and shared memcache keys in staging (preview / verify) and sandbox environments to **prevent production cache pollution**.

We can consider using the same approach for devboxes.

Replaces a quick fix from #5765

@Wikia/platform-team 
All other parts of the key use : as a separator.   I don't think it makes any difference functionally, unless something is trying to read/sort/normalize memcache keys externally?