Implement query for NYX_OS_INFO_WEBOS_PRERELEASE

:Release Notes:
The OSInfo module now returns a build-provided value for the
NYX_OS_INFO_WEBOS_PRERELEASE query.

:Detailed Notes:
Each <distro>.conf will make an assignment to WEBOS_DISTRO_PRERELEASE
variable. When it's unset (i.e., "" has been assigned to it), the
NYX_OS_INFO_WEBOS_PRERELEASE query should return ""; otherwise, it
should return its value.

:Testing Performed:
Tested query with and without WEBOS_DISTRO_PRERELEASE set before
building nyx-modules.

:QA Notes:

:Issues Addressed:
[GF-2855] nyx-modules: Replace NYX_OS_INFO_WEBOS_PRERELEASE with a real
          value

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>

Change-Id: I2e5601265ead260ebb9b0e8c1635730cc452ca0e
Reviewed-on: https://g2g.palm.com/3340
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>