Fix race in get_network(s) in OVS plugin

Load network bindings eagerly with networks.
Otherwise a different db query could try to fetch network bindings
for already deleted networks. The issue is reproducible with
concurrent tempest network API tests.

Closes-Bug: 1263686
Change-Id: I0521ab162220c66a62491ff8e7e4a9f6d7bef6c4
Rename nicira configuration elements to match new naming structure

- Every config item prefixed with nvp is prefixed with nsx
- deprecation qualifiers are added to preserve bw compatibility
- nicira/nvp.ini is renamed to vmware/nsx.ini
- symlink nicira/nvp.ini is created to point to vmware/nsx.ini
- UT added to verify that nvp.ini and old config items can still
  parsed correctly; bw-compat will be dropped in Juno

Partial-implements blueprint nicira-plugin-renaming

Change-Id: I676b868e61064cc5ff17e2246e83ba5c5e4a3449
Merge "Fix race in get_network(s) in OVS plugin"
Merge "Rename nicira configuration elements to match new naming structure"
Enables quota extension on BigSwitch plugin

Adds quotas to supported extensions in
BigSwitch plugin and adds db migration script
to create quota tables for BigSwitch plugin.

Closes-Bug: #1266334
Change-Id: I255b318fde792ef59fe2cf456df4be87135c65cd