Changes for Sahara and Murano OSTF tests

We should allow running Sahara tests only when compute node has
at least 4G RAM. That should greatly reduce number of errors. So this
limit was added to OSTF tests for Sahara.

Method for checking minimal free RAM of compute node was created and
placed in class PlatformServicesBaseClass. Now OSTF tests for Murano
use this method.

Closes-Bug: #1334700

Change-Id: Ic9333edc73c668ea5b953bb9dfa7c209fe2e258a