mgr: check for unicode passed to "set_health_checks()"

mgr: check for unicode passed to "set_health_checks()"

in python2, in addition to `str`, a string could also be unicode
string. there is chance that user passes a unicode string to
`set_health_checks()`, so let's check for unicode string also.

in python3, all strings are `unicode` string, and `PyString_Check()` is
defined as an alias of `PyUnicode_Check()`, so we are fine with python3.

the wrapper method of `PyString_ToString()` returns a pair of converted
string and a bool. if it fails to detect a string, an empty string and
false are returned.

Signed-off-by: Kefu Chai <kchai@redhat.com>