Add explicit "roles.yml" support to testclusters

[tvernum]

Previously, within tests, the file "roles.yml" (that is used to define
security roles in a cluster) would need to be configured using
extraConfigFile. This is effective, but means that there can only be
a single source of security roles for the testcluster.

This change introduces an explicit "securityRoles" setting in
testclusters that will concatenate the provided files into a single
"roles.yml" in the config directory. This makes it possible for
testclusters itself to define standard roles as well as having each
test define additional roles it may need.

Relates: #81400

[elasticmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[tvernum]

In #81400 we're changing the privileges of superuser so that is can no longer do some of the things that the test infrastructure needs (e.g. delete system indices between tests).
To overcome that we need the test code to introduce a new role that has those privileges. However, there was no easy way for ElasticsearchNode to install a roles.yml file if the test itself also had one.

I could put a hack into ElasticsearchNode to recognise an extraConfigFile named "roles.yml" and manipulate it to add the additional role, but it seemed like a better idea to push the config into a first class property instead.

I've only changed 1 QA test in this PR. Once it's approved I can go through and change all the other tests as well and then we can consider making it an error to set roles.yml as an extraConfigFile.

[mark-vieira]

Couple of comments but otherwise LGTM. Thanks, Tim.

[mark-vieira]

@tvernum can we backport this to 7.17 as well? We want to keep all long term support branches as similar as possible WRT to build infrastructure since it makes our lives considerably easier down the road.