Fix for nested apps add-select widget

The previous version of murano add-select widget didn't work in case
there were both vanilla horizon add-select widget and custom murano
one. Avoid this by wrapping parsing json inside try .. catch block to
continue script execution in case vanilla add-select was found, also
simplify creating custom add-select widget a bit.

This patch relies on https://review.openstack.org/#/c/117021/6

Co-Authored-By: Ekaterina Chernova <efedorova@mirantis.com>
Closes-Bug: #1384178
Change-Id: Ia6d895e4bbeb1238815d2c4bb57d706da9655f45
Merge "Fix for nested apps add-select widget"