Allow changing of node memory quota

'curl -i -u <user>:<pass> -d memoryQuota=500 http://localhost:8091/pools/default' should work to change the per-node quota when run against a cluster. Currently it doesn't work and the error in the UI is:
Server error during processing: ["web request failed",

{path,"/pools/default"}

,

{type,exit}

,

{what, 'Changing the memory quota of a cluster is not yet supported.'}

,
{trace,
[

{menelaus_web,handle_pool_settings,2}

,

{menelaus_web,loop,3}

,

{menelaus_web,'-start_link/1-fun-0-',3}

,

{mochiweb_http,headers,5}

,

{proc_lib,init_p_do_apply,3}

]}]