Fixes arguments parsing when executing command

A regression was caused by changing the eventlet.subprocess.Popen
with the builtin subprocess.Popen (by using the nova.utils execute
method) without changing the way the arguments were parsed.

In the v1 volume utilities module, the execution args were parsed
separated by whitespaces, which is not allowed by the builtin
subprocess.Popen, causing a "not found" error.

This error is returned for example when attaching a volume, at the
point where iscsicli tool is used to login the iSCSI target or portal.

This patch fixes the issue by simply splitting the args.

Closes-bug: #1317180

Change-Id: Iee7d5de0dde8b68d8f2bab4214e9b6779ad9f722
Merge "Fixes arguments parsing when executing command"