Fix PY3 issue

- In PY3 the output of subprocess.Popen is a type bytes, but
strip() method requires a type str.

Story: 2008017
Task: 40669

Change-Id: I297c30044df9a94baa645a1a27de10bb49038440

monasca_agent/collector/checks_d/mysql.py

@@ -370,7 +370,7 @@ class MySql(checks.AgentCheck):
                                            'pid'],
                                           stdout=subprocess.PIPE,
                                           close_fds=True).communicate()[0]
-                    pslines = ps.strip().split('\n')
+                    pslines = ps.decode('utf-8').strip().split('\n')
                     # First line is header, second line is mysql pid
                     if len(pslines) == 2 and pslines[1] != '':
                         pid = int(pslines[1])