This adds the ENABLE_VOLUME_MULTIATTACH flag and if True
configures Tempest to run volume multiattach tests.
Note that due to https://bugzilla.redhat.com/show_bug.cgi?id=1378242
we can't run multiattach tests with the Pike UCA packages since
those include qemu 2.10 and libvirt 3.6, and the valid versions for
multiattach support with libvirt is qemu<2.10 or libvirt>=3.10.
Depends-On: I80c20914c03d7371e798ca3567c37307a0d54aaa
Depends-On: I158c6f20e3e6a24bd2e5299abbeb3fc5208e5885
Part of nova blueprint multi-attach-volume
Change-Id: I46b7eabf6a28f230666f6933a087f73cb4408348