powervm: Implement snapshot for local volumes

Snapshot local volumes by copying the logical volume on VIOS,
gzip the file, transfer to compute service node, and
finally upload to glance. PEP8 and string concatenation fixes.
Added utility to powervm.common for constructing UNIX file paths.
Added unit tests for new utility.

bp powervm-compute-enhancements

Change-Id: Ie6bafbeef75a9464457d78d2997be78346d3ab24
Merge "powervm: Implement snapshot for local volumes"