QA: add toggle for fewer common instance tests

Currently, the "common" instance tests (shutdown, rapi stopped
console, stopped modify, startup, rapi shutdown, rapi startup, list,
info, modify, rapi modify , console, rapi console, reinstall, rename,
rapi rename, rename and back, rapi rename and back, grow disk, reboot,
tags, cluster verify, rapi test instance, node list, job list) are run
4 times:

- one time for plain instance created via RAPI, using client
- one time for plain instance created via RAPI, without client
- one time for plain instance created via gnt-instance
- one time for DRBD instance created via gnt-instance

This makes the QA long, and is over-doing it (for non-full QAs): an
instance created via RAPI (either client) and via gnt-instance should
have the same parameters, but our duplicate tests do not ensure that,
just that instances behave OK.

This patch adds a toggle so that we can skip the common tests for
RAPI-created instances; the creation/deletion is still performed, but
all the other operations are not. This reduces the time of a "quick"
QA by ~20% (1h:15m → 1h).

The common tests will still be run (unconditionally) for
gnt-instance-created instances.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>