Since elastic#65905 Elasticsearch has determined the Java heap settings
from node roles and total system memory.

This change allows the total system memory used in that calculation
to be overridden with a user-specified value. This is intended to
be used when Elasticsearch is running on a machine where some other
software that consumes a non-negligible amount of memory is running.
For example, a user could tell Elasticsearch to assume it was
running on a machine with 3GB of RAM when actually it was running
on a machine with 4GB of RAM.

The system property is `es.total_memory_bytes`, so, for example,
could be specified using `-Des.total_memory_bytes=3221225472`.
(It is specified in bytes rather than using a unit, because it
needs to be parsed by startup code that does not have access to
the utility classes that interpret byte size units.)