Feature Request: make _has_memory_headroom function more configurable

[do3meli]

The function _has_memory_headroom is actually checking if the system has less than 95% of the total memory available free. If so it will return true and otherwise it will return false:

salt/salt/minion.py

Lines 2110 to 2116 in 24a4cfa

	mem = psutil.virtual_memory()
	if mem.percent > 95:
	log.warning(
	"Memory limit reached (Used: %s%%). Pausing queue processing.",
	mem.percent,
	)
	return False

The problem with this is, that salt-minion jobs get queued up on nodes that have a huge amount of memory still free. Let's say you have 2TB memory nodes. 5% memory headroom on such a system would be 102.4 GB. I guess that should be more than enough for salt ;-) Therefore i suggest to have the following to adjustments:

1. make the memory limit in percent configurable: instead of using 95 as hardcoded in the code substitute this with a variable that can be configured via minion config.
2. enhance the condition in a way that it is also possible to use a pre defined (via config of minion) number of memory that needs to be available (for example 5 GB). If that is set, i guess it would be preferred over the percent that is defined.