Fix broken formatting of processutils.execute log statement

The processutils.execute function will log a message about
each command it runs which looks like this:

   Running cmd (subprocess): /bin/echo hello world

In the following commit:

  commit cdcc19c1d78a4a88daabfb64b27abd4924aa442d
  Author: Brad Pokorny <bpokorny@us.ibm.com>
  Date:   Sun May 18 18:26:33 2014 +0000

    Mask passwords that are included in commands

The command is passthrough through logging.mask_password
first. Unfortunately this method expects a string as its
parameter, but is given a list of strings instead. This
causes it to stringify the list object. The result is that
the log message ends up being mangled to look like this:

   Running cmd (subprocess): [ ' / b i n / e c h o ' ,
   ' h e l l o   w o r l d ' ]

The execute method should have been masking the password
after turning the list of strings into a single string,
instead of before.

Change-Id: I994a3449a6a88e7ba1dd24b1e65183c018e4c3a3
Merge "Fix broken formatting of processutils.execute log statement"