Introduce a way to configure total amount of RAM available

[michaelklishin]

Originally reported as #205. There's relatively little demand for this but it was brought up as a follow-up improvement to #1223 for some cases.

Currently nodes determine the total amount of RAM available using an OS-specific tool such as /proc/meminfo. In some cases it would be more convenient for the user to provide a value e.g. in the config file.

Proposed config key currently is rabbit.total_memory_available_override_value (a positive integer).
When not set (which is the default), the current OS-specific metric mechanism will be used.