VCS types: avoid assumptions on Heptapod defaulting.

Before this change, these tests were just assuming that the `hg_git`
VCS type was always allowed in instance-wide settings and that it was
further the default VCS type. This new version should be able to
take care of all cases.

Instead of asserting the resulting VCS type based on expected default,
we now force the wished VCS type and merely assert that we get it back
on the created project.

See heptapod!272 and heptapod#523
10 jobs for !98 with topic/default/default-vcs-type-native in 22 minutes and 14 seconds (queued for 4 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed flake8 #247004

00:00:14

 
  Test Heptapod
passed docker-fully-native-inside #247011

00:18:39

passed docker-fully-native-remote-0 #247012

00:15:24

passed docker-fully-native-remote-1 #247013

00:21:58

passed docker-inside #247005

00:19:23

passed docker-native-inside #247008

00:15:56

passed docker-native-remote-0 #247009

00:18:52

passed docker-native-remote-1 #247010

00:20:58

passed docker-remote-0 #247006

00:18:26

passed docker-remote-1 #247007

00:21:17