Cast cluster health keys and values to strings before attempting to utf8 encode them

This check is currently broken with this error:

Traceback (most recent call last):
File "/usr/lib/nagios/plugins/check_elasticsearch.py", line 184, in <module>
sys.exit(main())
File "/usr/lib/nagios/plugins/check_elasticsearch.py", line 180, in main
return check_elasticsearch(options)
File "/usr/lib/nagios/plugins/check_elasticsearch.py", line 158, in check_elasticsearch
_format_health(cluster_health)))
File "/usr/lib/nagios/plugins/check_elasticsearch.py", line 83, in _format_health
v.encode('utf8', 'ignore'))
AttributeError: 'int' object has no attribute 'encode'

Change-Id: I242e74dc84408265004b4dc7d6f7739d53a4774b