Isolate tests from user/system config

git-review behavior depends on git local, global and system configs.
tests have the same dependencies against them but only manage git local
config which implies the config expected by tests could not be the real
one because of git global and system configs.

The same trouble concerns gitreview global and system config files.

This change allows to run git-review in local mode (only using git and
gitreview local config files) by defining the environment variable
"GITREVIEW_LOCAL_MODE". This mode is used by tests to isolate them from
git and gitreview global and system configs. Mocking can not be used as
functional tests call not git-review code but git-review system command.

Change-Id: I598a284aec9e55f9618ab026c55ef0435e370d69
Closes-Bug: #1393192