Replace underscore by spaces in PLATFORM

platform is used to craft the Jenkins jobs name which does not accept
spaces while Sauce Labs require spaces.  Simply replace all underscores
by spaces using some bash magic.

${VARIABLE//_/ } can be read as:

In VARIABLE, replace all occurences (//) of underscore by (/) space.

Using double braces for escaping.

Test:

#!/bin/bash
for PLATFORM in 'Windows_XP' 't__t' 'Linux' 'Windows_9'; do
echo "Input:    $PLATFORM"
export PLATFORM=${PLATFORM//_/ }
echo "Platform: $PLATFORM"
done;

Outputs:

Input:    Windows_XP
Platform: Windows XP
Input:    t__t
Platform: t  t
Input:    Linux
Platform: Linux
Input:    Windows_9
Platform: Windows 9

Change-Id: I14616b99e6cf62397285ef80c63af79aaea96c32