Add documentation of Resource Manager models

src/docs/user/references/Variables.adoc

@@ -140,6 +140,7 @@ Examples: PA:LIST(a,b,c) will accept "a", "b", "c" but no other value.
 * *PA:REGEXP(pattern)* : variable syntax must match the regular expression defined in the pattern. The regular expression syntax is described in class https://docs.oracle.com/javase/7/docs/api/index.html?java/util/regex/Pattern.html[Pattern]. +
 Examples: PA:REGEXP([a-z]+) will accept "abc", "foo", but not "Foo".
 * *PA:MODEL_FROM_URL(url)* : variable syntax must match the model fetched from the given URL. This can be used for example when the model needs to represent a list of elements which may evolve over time and is updated inside a file. Such as a list of machines in an infrastructure, a list of users, etc. +
+See <<_variable_model_using_resource_manager_data>> for premade models based on the Resource Manager state. +
 Examples: PA:MODEL_FROM_URL(file:///srv/machines_list_model.txt), if the file machines_list_model.txt contains PA:LIST(host1,host2), will accept only "host1" and "host2", but may accept other values as the machines_list_model file changes.
 * *PA:CRON* : variable syntax must be a valid cron expression as defined in the  http://www.sauronsoftware.it/projects/cron4j/manual.php#p02[cron4j manual]. +
 Examples: PA:CRON will accept "5 * * * \*" but not "* * * *" (missing minutes sub-pattern).
@@ -180,6 +181,37 @@ For example, a variable `MY_OPTIONAL_INTEGER` defined as the model `PA:INTEGER?`
 
 All the available model syntaxes, except `PA:NOT_EMPTY_STRING` support to be defined as *optional*.
 
+====== Variable Model using Resource Manager data
+
+ProActive Resource Manager provides a set of REST endpoints which allow to create dynamic models based on the Resource Manager state.
+
+These models are summarized in the following table. Models returned are PA:LIST types which allow to select a value in ProActive portals through a drop-down list.
+The list always contain an empty value choice.
+
+.Resource Manager Models
+[%header,cols=4*]
+|===
+|Metric Name
+|Description
+|Model Syntax
+|Example returned data
+
+|Hosts
+|All machine host names or ip addresses registered in the Resource Manager
+|`PA:MODEL_FROM_URL(${PA_SCHEDULER_REST_PUBLIC_URL}/rm/model/hosts)`
+|PA:LIST(,try.activeeon.com,10.0.0.19)
+
+|Node Sources
+|All node sources registered in the Resource Manager
+|`PA:MODEL_FROM_URL(${PA_SCHEDULER_REST_PUBLIC_URL}/rm/model/nodesources)`
+|PA:LIST(,Default,LocalNodes,GPU,Kubernetes)
+
+|Tokens
+|All tokens registered in the Resource Manager (across all registered ProActive Nodes). See <<_node_access_token>>.
+|`PA:MODEL_FROM_URL(${PA_SCHEDULER_REST_PUBLIC_URL}/rm/model/tokens)`
+|PA:LIST(,token1,token2)
+|===
+
 ====== Spring Expression Language Model
 
 The `PA:SPEL(expr)` model allows to define expressions able to validate a variable value or not. Additionally, this model can be used to validate multiple variable values or to dynamically update other variables.