Config file for commonly applied flags

( This can be thought of as a pre-cursor to MB-9199 or its minimum viable product.)
Very often we are advising customers to apply a set of bespoke flags to their environments. Common examples include:

"+swt low" for better distribution of erlang job queue
"-c nnnnnn" to increase memcahced's maximum number of connections
"-t nn" to increase memcached's thread count.

Right now these flags are applied by either (1) hacking the init.d script, (2) applying a curl command on the fly and, or (3) replacing the memcached binary with a wrapper script. These methods pose the following problems:

(1) Requires the customer to perform some undocumented commands
(2) Such customisations can be easily overlooked when debugging a production deployment
(3) They are not easily portable to new clusters
(4) They can be lost or interfere with the upgrade process

This request is to allow the setting of these parameters via a standard local configuration file. This configuration file would need to be respected on upgrade. Ideally these options would also be exposed via the GUI but I appreciate that is a more involved change and could feature as part of MB-9199.