Add Napalm get_vlans() method

[jefvantongerloo]

Reference issues
None

Fixes
Implements the get_vlans() method based on the official Napalm documentation:

get_vlans()Return structure being spit balled is as follows:
- vlan_id (int)
   - name (text_type)
   - interfaces (list) 

Example output:

{
   10: 
   {
   "name": "default",
   "interfaces": ["GigabitEthernet0/0/1", "GigabitEthernet0/0/2"]
   },
   11: 
   {
   "name": "vlan11",
   "interfaces": []
   }
}

[jefvantongerloo]

get_vlans() method and test getters not present in Napalm version 2.3.1, needs Napalm 3.0+ upgrade compatibility.

Reference issue: iget_vlans() method and test getter not present (Napalm < 3.0) #35
Depends on PR: Make compatible with Napalm 3 #30