Enlarge Cinder-Volume container

Cinder requires temporary working space to convert images. This patch
exposes cinder_volume_lv_size_gb to the user config file, so the user
can decide how large the cinder volumes container should be based on
available space and the size of images that will need to be converted.

cinder_volume_lv_size_gb is used to override container_lvm_fssize in
group_vars/cinder_volume. Simple enough but doesn't work because
templated variables (or indirect variables) are not expanded when
accessed via hostvars[] see: ansible/ansible#7844. In order to work
around that, I have eliminated hostvars[] usage from the container
creation mechanism. This may have positive speed implications as the
limit of container creation parallelism is now forks rather than number
of hosts. However it does make this change larger than a small bug fix.

Also note that this patch makes use of delegate_to, so specific ansible
versions must be used to avoid ansible/ansible#8705. Our requirements
file currently specifies a version before this bug was introduced.

There are two commits in this PR as one is the actual bugfix, the other
is infrastructure changes required for that bugfix to work. Also only
the bugfix may be needed if the upstream bugs are fixed.

Closes-Bug: #1399427
Change-Id: I2b5c5e692d3d72b603fdd6298475cb76c52c66df
Merge "Enlarge Cinder-Volume container"