Better argument default list handling

If a function argument is defined as a list type and it's a string then
split on ',' characters.  This will allow list types to be specified in
config ini files like.

[cmd:nova.boot]
security_groups = default,rctest-node

Change-Id: I84073b2987aee004a651a496fa65c1c1f5fff6ed
Merge "Better argument default list handling"