Make memory_limit_bytes available as optional bosh property

The code allows both a configurable % and byte limit for memory. Specifically the byte limit makes sense when you want
to run log-cache on a really large VM, colocated with other things, but you don't want it to take up too much
of the VM's memory.

This optional property will do nothing until set, at which point it will override the memory % limit, as documented.

Author: dsboulder

jobs/log-cache/spec

@@ -43,6 +43,9 @@ properties:
     description: "Percentage of system memory to use for the cache. Must be an integer."
     default: 50
 
+  memory_limit_bytes:
+    description: "Overrides memory_limit_percent with a specific size cache in bytes."
+
   max_per_source:
     description: "The maximum number of items stored in LogCache per source."
     default: 100000

jobs/log-cache/templates/bpm.yml.erb

@@ -25,6 +25,9 @@ processes:
   env:
     ADDR:        "<%= ":#{p('port')}" %>"
     MEMORY_LIMIT_PERCENT: "<%= p('memory_limit_percent') %>"
+<%- if_p('memory_limit_bytes') do |memory_limit_bytes| %>
+    MEMORY_LIMIT_BYTES: "<%= memory_limit_bytes %>"
+<% end -%>
     MAX_PER_SOURCE: "<%= p('max_per_source') %>"
     INGRESS_BUFFER_SIZE: "<%= p('ingress_buffer_size') %>"
     INGRESS_BUFFER_READ_BATCH_SIZE: "<%= p('ingress_buffer_read_batch_size') %>"