tests: add earthquake repository as a submodule

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
tests/earthquake: add tgt as a submodule

It will make preparing earthquake ready tgt binary easily.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
add a new option --enable-earthquake for enabling earthquake ready sheep and dog

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
tests/earthquake: testing framework and first test case

This patch adds a slack testing framework for earthquake and first
test case.

The test case revealed the problem related to snapshot. Current
sheepdog creates a snapshot of existing VDI with the two command:
1. let the existing VDI snapshot
2. create a new writable VDI

1 and 2 cannot be done atomically (if we want to do it, we need a
transaction mechanism). If tgtd detects the working VDI became
snapshot and tires reload a new writable one before 2, it hangs and
iSCSI initiator cannot do nothing (just wait timeout).

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
update MAINTAINERS file for earthquake testing stuff

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>