check_ceph_osd_db: Decode bytes output to string

[lbausch]

Avoids TypeError: cannot use a string pattern on a bytes-like object when executing re.findall().

Popen.communicate() returns bytes if the streams weren't opened in text mode, which is not the case here, as far as I can tell. See https://docs.python.org/3/library/subprocess.html#subprocess.Popen.communicate

[jg-basis]

Noted this issue and it fixed the problem for me.