Follow up on the #6809 (Reserve memory for host) to add global settings for upper/lower limit

##### ISSUE TYPE

* Improvement Request

##### SUMMARY

In the #6809 PR, @wido suggested to add a new global settings to bound the value of `host.reserved.mem.mb` global setting. Due to the tight schedule, we planed to add these setting in a new PR with different milestone.

Suggested **global** settings are:

| Name | Description | Default value |
|---------------|------------|--------------|
| host.reserved.mem.mb.lower.bound | Sets a lower bound for reserved memory in megabytes. It is not possible to reserve less than this amount for any host. | 0|
| host.reserved.mem.mb.upper.bound | Sets a upper bound for reserved memory in megabytes. It is not possible to reserve more than this amount for any host. The default value is 1 Tbytes | 1048576|