[title]
Ensure memory usage is within the threshold

[description]
The check examines the RAM usage of the system. If not enough RAM is available
system will slow down and become much more unresponsive and difficult or even
impossible to use.

[param warn_limit]
Memory usage (in percent) at which to raise a warning exception.
Valid values are integers in the range of  1 to 100

[param critical_limit]
Memory usage (in percent) at which to raise a critical exception.
Valid values are integers in the range of  1 to 100
