veyron/lib/{modules,exec}: tidy up env var handling.
Tidy up the handling of environment variables in particular to
make working with and implementing the node manager and associated
tests easier.
The exec package now unsets the VEYRON_EXEC_VERSION env var when
it no longer needs to avoid having it be accidentally passed on
to other process that are are not necessarily started by this
package.
The modules shell type now has a 'CommandEnvelope' method that can
be used by the node manager when constructing envelopes for use
with the app manager.
Change-Id: I86a896557bdf6b87f39523fb7399453cf7d2a0fe
8 files changed
