Bug 8071 - Do not fail on modules with invalid revision during

Bug 8071 - Do not fail on modules with invalid revision during
           resolution of ietf-netconf-yang-library/modules-state node

After parsing xml with invalid revision string it failed instead
of returning only modules with correct revision.

It reads an xml, creates a new xml string using only modules with
correct revision and logs the ones that has wrong revision. New xml
string will be parsed to normalized node.

Change-Id: Ie51163864f3547560840cb7d52ea8762437a6b1e
Signed-off-by: miroslav.kovac <miroslav.kovac@pantheon.tech>