Calls to update_cache prior to addVlan in circuit.cgi causes provisioning timeout

[jonstout]

If any switch process become unresponsive, calls to update_cache prior to addVlan in circuit.cgi causes provisioning timeout (completion of call requires a response or timeout from all switch processes).

This provisioning timeout is not observed in vrf.cgi as update_cache is never called. Instead, when addVrf is called, the VRF object is loaded from the database on first use.

Update addVlan to use this same strategy. The effect should be faster provisioning and ability to provision when irrelevant switch processes are unresponsive.