Add `ios_vrf_global` model

[Ruchip16]

This PR adds argspec model for ios_vrf resource module.

FYI: adding device configuration for reference

admin#show running-config vrf
Building configuration...

Current configuration : 1707 bytes
vrf definition test
 vnet tag 34
 description This is test VRF
 ipv4 multicast multitopology
 ipv6 multicast multitopology
 rd 10.2.3.4:300
 vpn id 3:4
 route-target export 23.1.3.4:400
 route-target import 123.3.4.5:700
 !
 address-family ipv4
  import ipv4 multicast 89 map import
  export map testing-map
  export ipv4 multicast 345 map single
  mdt auto-discovery pim pim-tlv-announce mdt-hello-enable
  mdt auto-discovery receiver-site
  mdt strict-rpf interface
  mdt default ingress-replication
  mdt direct
  mdt mtu 789
  mdt overlay bgp shared-tree-prune-delay 200
  mdt overlay bgp source-tree-prune-delay 23
  mdt overlay use-bgp spt-only
  snmp context test user ruchi
  maximum routes 4 1
  inter-as-hybrid next-hop 1.2.3.4
  bgp next-hop Loopback23
  protection local-prefixes
 exit-address-family
 !        
 address-family ipv6
 exit-address-family
 !        
 address-family ipv4 multicast
  topology base
   route-replicate from unicast mobile route-map point
   route-replicate from unicast odr route-map thik
   route-replicate from unicast ospf 34 route-map someing
   route-replicate from unicast rip route-map test
   route-replicate from unicast static route-map rest
   !      
   route-replicate recursion-policy destination
   !      
   route-replicate-distance from unicast 67
  snmp context iosx community ipv6 RO ghj
   snmp context iosx user ruchi credential
   bgp next-hop Loopback34
   use-topology unicast base
  !       
  topology test
   snmp context iosvrf user alex access func
   all-interfaces
   use-topology unicast base
  !       
 exit-address-family
!         
!         
!         
end