Prefer git-config over git-review config files

git-review gets its configuration from multiple sources (by priority):

 * command line options
 * git-config
  * options: username, rebase
  * with local/global/system config
 * git-review config files
  * options: scheme, host, port, project, defaultbranch/remote/rebase
  * locally using .gitreview
  * globally using ~/.config/gitreview/git-review.conf
  * system using /etc/git-review/git-review.conf

.gitreview file is commonly provided versioned in the git repo, where
other git-review config files and git-config is done by developers. It
implies for developers multiple places to define local/global/system
configuration options but some can only be changed by updating
.gitreview (scheme/port...) which is under versioning.

This change proposes to use as configuration sources (by priority):

 * command line options
 * git-config for developer local/global/system config
  * options: username, rebase and all .gitreview options
 * .gitreview local file for repo provided specific config

and deprecates git-review global/system files in order to reduce
configuration sources and allow to overload .gitreview configuration
without updating it.

Change-Id: I24aba0fa089de57d51a79049d9c192f76d576f69
Merge "Prefer git-config over git-review config files"