Reduce Linux Pressure (PSI) storage cost

[pavlozt]

These are very promising indicators that can replace Load Average in future, but their implementation turned out to be too expensive for storage. Being attached to almost all hosts, it is the heaviest metric overall my infrastructure.
I propose changes to reduce the storage cost:

• there is no point in storing original texts elements if dependent elements are calculated correctly.
• The polling frequency should be the same as for regular metrics in Linux. One minute is just enough.

Also, I'm specifically create templates in yaml format for version 7.0. Store defaulted to 31d as usual in 7.0 version

Original description and README preserved for 7.0 .
Added link to official information about PSI.