(maint) Wrap windows PE installation in `cmd /C`

When cygwin converts the posix-style commandline that bash provides to
a windows-style one for calling msiexec, it adds quotes around some
arguments. These quotes cause msiexec to fail. By wrapping the msiexec
invocation in `cmd /C`, we use the windows command interpreter to
handle the command line instead of bash/cygwin. This avoids the
translation issues introduced by cygwin.
(maint) Install PE on windows synchronously

Previously, beaker would call msiexec in a way that would cause it to
run asynchronously. This worked pretty much by chance, because beaker
did enough other stuff before trying to run puppet on the Windows
machine.

This wraps the msiexec call in `start /w`, which waits for the
installation to finish before continuing. This ensures that puppet is
fully installed before beaker continues.
Merge pull request #185 from branan/use_cmd_for_msiexec

(maint) Wrap windows PE installation in `cmd /C`