Multiple fixes for ZFS support

• Import pools from /etc/zfs/zpool.cache if exist
• Replace eval with while to support retrying password, previously one failed attempt will drop into emergency shell
• Fix: Enclose zpool list command output in double quotes
• Fix: _root_pool has empty value. See #14 (closed).

Without the quote, [] treats the output as an expression, not as a string, thus breaks the test against string active.

@ncopa