Description of problem:
<reservesys/> is silently discarded from job XML.

Version-Release number of selected component (if applicable):
22.0rc1 (does not affect any released version)

How reproducible:
easily

Steps to Reproduce:
1. Submit a job which uses <reservesys/> after the <task/> elements in the <recipe/>
(see https://beaker-project.org/docs/user-guide/system-reservation.html#using-the-reservesys-element)

Actual results:
Beaker does not reserve the system at the end of the recipe.
If you clone the job, the <reservesys/> element is missing.

Expected results:
Beaker should reserve the system at the end of the recipe.
The <reservesys/> element should be preserved when the job is cloned.

Additional info:
Regression in recipe XML parsing due to the lxml conversion. <reservesys/> is missing from complete-job.xml so parsing of <reservesys/> is not covered by any tests.