Add execute wrapper to brick LVM code.

Add an execute wrapper to the brick lvm code and
enable the ability to pass in a desired executor other
than just using the default common/processutils executor
if there's some reason to do so.

This will enable the switch of the cinder/volume/drivers/lvm.py
driver to continue using cinder/utils execute until we switch over
to common/processutils and also helps immensely with the tests.

Change-Id: Iaecba475debb2100d0c40b5e54ece6d4420271c5
Merge "Add execute wrapper to brick LVM code."