Make platform all lower case

The job template will vary by platform. Zeljkof confirmed that Saucelabs
platform can be set all lower case which saves us from rename the jobs
from linux to Linux.

Change:

    -export PLATFORM='Linux'
    +export PLATFORM='linux'

Change-Id: I31624526aa5c7c2f2e57ea30fadd7aeaa9ed45e1