Check that specified sys user is part of oozie impersonation group

## Background
In order to impersonate a user, both the oozie proxy user and user to be impersonated have to be part of the configured functional/security group.

## Feature
Query LDAP in order to validate that specified user is indeed part of the impersonation group.

Edit:
Rather than querying ldap let's just catch the exception and display a sensible error message for now.