Add ability to get common or node part of context in lcm

[OpenStack Infra]

https://review.openstack.org/368661
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit 8fbcf681798144d11b650b05b47f6f2e56cbd081
Author: Dmitry Guryanov <email address hidden>
Date: Fri Sep 9 13:23:57 2016 +0300

    Add ability to get common or node part of context in lcm

    The main purpose of this commit is to have an ability
    to split configuration file astute.yaml into common
    and node parts. Common part is huge and we will
    dump it once and also there will be only one instance
    of this data in RAM which saves a lot of memory when
    you run deploy on many nodes (>100).

    This patch adds two new variables to context, which can be
    used in yaql_exp: $node and $common for node and common
    parts of the context. Functions changed, changed_all,
    changed_any, added, deleted don't work for these variables.

    DocImpact
    Change-Id: I56bf982652a5dc27882e4a401ca9ec124899fed7
    Partial-Bug: #1596987