webadmin: Extract code of network role columns

NetworkRoleColumnHelper is introduced to re-use the construction of
value and tool tip in various network role columns (Network Main tab,
Network Cluster sub tab and Cluster Network sub tab).

Change-Id: I36ea65cce1ec3718838d5d2f8252adcc996b2fa7
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>