Beaker payload definition - Add support for reservesys XML element

[StykMartin]

Is your feature request related to a problem? Please describe.
At this moment it is not possible to define reservesys XML element in Pinfile per recipe.
This can effectively replace reservesys task in some cases.

Definition

<element name="reservesys">
	<a:documentation xml:lang="en">
	If this element is specified, the system will be reserved at the end of the recipe when all tasks have completed execution.
	</a:documentation>
	<optional>
		<attribute name="duration">
			<a:documentation xml:lang="en">
			Reserve for the specified duration in seconds. If not specified, this defaults to 86400 seconds or 24 hours.
			</a:documentation>
		</attribute>
	</optional>
	<optional>
		<attribute name="when">
			<a:documentation xml:lang="en">
				Reserve the system only under certain conditions. The default value "always" will reserve the system unconditionally. The value "onwarn" will reserve the system if any task reported a Warn or Fail result, or if the recipe aborted. The value "onfail" will reserve the system if any task reported a Fail result, or if the recipe aborted. The value "onabort" will reserve the system if the recipe aborted.
			</a:documentation>
			<choice>
				<value>always</value>
				<value>onwarn</value>
				<value>onfail</value>
				<value>onabort</value>
			</choice>
		</attribute>
	</optional>
</element>

Describe the solution you'd like
Sometimes we can't rely on harness to keep reservesys running therefore Beaker implemented reservesys without harness.

reservesys XML element should be used instead of task if defined.

Describe alternatives you've considered
No other options here

Additional context
Add any other context or screenshots about the feature request here.