Instead of using empty strings for optional values, change the data type
to Optional[String] to ensure that either a valid String is provided
or no value is passwd.
A side effect is that these parameters cannot be set explicitely undef
by the end user if they have a default value, and $poudriere::zpool is
concerned by this. But since this default value is bogus, change it to
undef to avoid the issue.
Fixes parts of #52