Add a few tests for cinder multiattach

This adds basic compute API tests for testing a multiattach volume in
Cinder being attached to multiple instances in Nova and then
detaching them.

This change depends on the nova series that adds the support to nova's
libvirt driver.

Devstack change I46b7eabf6a28f230666f6933a087f73cb4408348 is used to
enable this test when using the libvirt driver.

Depends-On: I02120ef8767c3f9c9497bff67101e57e204ed6f4
Depends-On: Iac67f112b0dc9353c6a66e6fbc81cc8324a2b37c

Part of nova blueprint multi-attach-volume

Change-Id: I80c20914c03d7371e798ca3567c37307a0d54aaa
4 files changed