Fix encoding issues in nested output

This fixes #15542

Conflicts:
	salt/output/nested.py
Add missing import
Support unicode in highstate "changes" output

Since it invokes the nested outputter, which has been recently updated
to support unicode, the format string must be explicitly unicode.
Merge pull request #15855 from terminalmage/bp-15546-2014.1

Backport #15546 to 2014.1