Fix toggling templates

[xmj]

When temporarily marking a template as istemplate=no for maintenance (eg. starting it), the zfs dataset is copied from templates/ to jails without renaming it to its uuid.

This will break functionality in wicked ways, such as making iocell get state <jailname> always return down.

Steps to reproduce:

iocell create tag=testjail release=12.0-CURRENT
iocell set istemplate=yes testjail
iocell set istemplate=no testjail
zfs list |grep testjail   # note the datasets not containing uuids
iocell start testjail
iocell get state testjail