Replace use of master in Windows Dockerfile (#1143)

The `master` alias for the `aggregator` command was removed in #1137,
however this alias was still in use in the Windows Dockerfile. Replace
the use of this with the new command name `aggregator`.

Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>

DockerfileWindows

@@ -17,4 +17,4 @@ MAINTAINER John Schnake "jschnake@vmware.com"
 
 ADD BINARY /sonobuoy.exe
 WORKDIR /
-CMD /sonobuoy.exe master --no-exit -v 3 --logtostderr
+CMD /sonobuoy.exe aggregator --no-exit -v 3 --logtostderr