Fix riemann-health memory reporting when using ZFS on Linux

[smortex]

On Linux, memory used by the ZFS ARC is reported as used while in fact
it mostly consist of cached data that can be reclaimed by the operating
system if necessary.

If the system use ZFS, gather the ARC statistics to compute the size
of the ARC that can be evicted and do not count this as used memory.
This fix the obviously wrong reporting we can see on systems running
ZFS.

Containers running on nodes with ZFS can access the hosts ARC
statistics from the container, resulting in wrong memory reporting. To
avoid this, we detect if we are running from a container and skip
reading the ARC statistics in that case.

[smortex]

Here is a graph of a newly provisioned system showing the issue:

Initially, 2.5% of the memory is in use. At 8:50, we initiate the transfer of a large amount of data from another machine. Data written to disk quickly fill the ZFS ARC and the memory usage plateau at 53% of memory used. In fact, the workload has not really changed compared to the initial situation, and memory usage was expected to be the same, so I started to dig in this to find out what was wrong.

I first tried to follow what htop does by substracting the size of the ARC minus the minimum size of the ARC from used memory, but the result was still jumping up and down as files where sent, which did not look great. I then reworked the code and removed the "evictable" memory metrics which seems to be more reasonable (on the left, ZFS was not loaded and there was no ARC at all, so we expect the used memory when ZFS is in used to be slightly higher than when ZFS is not installed):

[jamtur01]

LGTM