kernel: recognize flavor key

kernel: recognize flavor key

Since commit 7da76fa78f63 ("Use kernel_flavor when polling
gitbuilders"), teuthology-suite appends 'flavor: basic' to kernel task
config.  This broke all scheduled kernel suites, because
normalize_config() treats unknown keys as role types and when it
doesn't find a valid one it bails and returns {}, which means "do
nothing".

Fix this temporarily by adding flavor to a list of recognized keys.
This is wrong a bunch of levels, but it fixes a regression while
sticking to the way normalize_config() currently works.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>