libvirt: Add new option to enforce multipath volume connections

Currently, even when [libvirt] volume_use_multipath is set to True,
volume attachment silently falls back to single path if multipathd is
not running in the host. This sometimes prevents operators from
noticing the misconfiguration, until they face any issue caused by
missing redundancy.

Introduce the new [libvirt] volume_enforce_multipath option, which
makes the attachment process fail if multipathd is not running.
A similar parameter (enforce_multipath_for_image_xfer) was already
implemented in cinder and this change follows how the parameter is
implemented there.

Also add the check in init phase to detect lack of mulitipath daemon
during initializing driver.

Min version of os-brick has to be bumped due to the interface change
made by 8d919696a9f1b1361f00aac7032647b5e1656082 .

Implements: blueprint enforce-multipath
Change-Id: I828de70ca7b343a4562ace4049d2b3857dbf900a

requirements.txt

@@ -49,7 +49,7 @@ rfc3986>=1.2.0 # Apache-2.0
 oslo.middleware>=3.31.0 # Apache-2.0
 psutil>=3.2.2 # BSD
 oslo.versionedobjects>=1.35.0 # Apache-2.0
-os-brick>=6.0 # Apache-2.0
+os-brick>=6.10.0 # Apache-2.0
 os-resource-classes>=1.1.0 # Apache-2.0
 os-traits>=3.3.0 # Apache-2.0
 os-vif>=3.1.0 # Apache-2.0