[TEST] more assertAcked in SharedClusterShapshotRestoreTests
Add plugin class space isolation

Each plugin is now loaded in its own classloader to prevent class
conflicts when loading different versions of the same library. It
 is enabled by default and is configurable through
 `plugins.isolation` settings .Additionally, each plugin can
change its own isolation through the `isolation` property in
`es-plugin.properties`- if not specified, the global setting in
ES applies.

Closes #5261
[TEST] Add version and master_node parameter to _cluster/state
[TEST] Dynamically extract node ID for test
fix failing IsolatedPluginTests (change numNodes to 0)

(cherry picked from commit f26a369a78ade032df44fcf331f378644f74b9ea)
[DOCS] Fixed added[] tag version number
make test assertions relative in IsolatedPluginTests

relates to #5621
[TEST] skip explain reroute REST test until param substitution is implemented