Unset LD_LIBRARY_PATH before executing commands (exec api)

New environment-setup-*-agl-linux file, used to set environment
variables to use an SDK, now checks if LD_LIBRARY_PATH is set or not.
And not empty LD_LIBRARY_PATH is considered as misconfigured and
prevent to use SDK.
So unset by default LD_LIBRARY_PATH env variable and allow to disable
this unset by setting ldLibPathNoReset field of ExecArgs command.

For info, reported error message when call environement-setup-* script:

Your environment is misconfigured, you probably need to
'unset LD_LIBRARY_PATH' but please check why this was set in the
first place and that it's safe to unset.
The SDK will not operate correctly in most cases when LD_LIBRARY_PATH
is set.
For more references see:
 http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html#AEN80
 http://xahlee.info/UnixResource_dir/_/ldpath.html

Change-Id: I386b92b04ebd33db39ac617acffa4e15b21213ad
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>