devel/review: take default reviewer from environment
devel/review: take default reviewer from environment

When adding a 'Reviewed-by'-line, also add the reviewer
passed through the REVIEWER environment variable. In that
way, adding the reviewer for patch series all reviewed
by the same person gets easier.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

bitemyapp/ganeti,mbakke/ganeti,onponomarev/ganeti,dimara/ganeti,yiannist/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,andir/ganeti,ganeti/ganeti,apyrgio/ganeti,andir/ganeti,andir/ganeti,leshchevds/ganeti,grnet/snf-ganeti,ganeti/ganeti,bitemyapp/ganeti,leshchevds/ganeti,apyrgio/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,grnet/snf-ganeti,yiannist/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1