UrlRepo: add support tests from url and tar

Since LAVA is not designed as a build farm, it will be used to
work with various build farm together.
So, it will be very useful to support tests/images from different sources.

In the current LAVA, LAVA supports tests from git/bzr and inline.
Support for tar and url is planned(not usable yet).

Now, with this patch, support for tar and url could be implemented.

Besides, the followings were also added in this patch.
- corresponding sample job and unit test
- Remove unused import(import indices) in testdef

Signed-off-by: Liu Wenlong <liuwl.fnst@cn.fujitsu.com>
15 jobs for test_from_tar in 8 minutes and 24 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #44600
amd64
dispatcher-debian-10

00:02:53

passed #44598
amd64
dispatcher-debian-9

00:03:14

passed #44601
amd64
server-debian-10

00:01:54

passed #44599
amd64
server-debian-9

00:02:13

 
  Analyze
passed #44603
amd64
black

00:00:26

passed #44605
amd64
code_quality

00:00:21

passed #44604
amd64
codestyle

00:00:33

passed #44607
amd64
job-schema

00:00:39

passed #44606
amd64
pylint

00:02:01

passed #44602
amd64-dind allowed to fail
sast

00:01:55

 
  Build
passed #44610
amd64
amd64/pkg-debian-10

00:01:08

passed #44609
amd64
amd64/pkg-debian-9

00:01:24

passed #44608
amd64
doc

00:01:13

 
  Docker
passed #44611
amd64-dind
amd64/lava-dispatcher

00:00:46

passed #44612
amd64-dind
amd64/lava-server

00:01:43