YARN-11925. Fix the configurability of RM webservice class

[p-szucs]

Change-Id: I108120c5407ed103adfcfcb469c44c7ed3de9999

Description of PR

RM webservice class is not configurable in trunk with the "yarn.webapp.custom.webservice.class" property since the upgrade to Jersey 2. The patch

• registers webservice class that is configured in the above property
• removes package scan on "org.apache.hadoop.yarn.server.resourcemanager.webapp" to avoid registering RMWebServices class at the same time

How was this patch tested?

Tested on a cluster with custom webservice class

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[K0K0V0K]

Thanks @p-szucs for the fix!
LGTM!