[rfr] Add Provider Option for Pool Creation

[jtopjian]

This commit adds the ability to specify a provider when creating a
LBaaS pool.

According to hashicorp/terraform#5545, a Provider option can be sent during Pool creation. neutron help lb-pool-create supports this as well as:

neutron --debug lb-pool-create --lb-method ROUND_ROBIN --name foo --protocol HTTP --subnet-id 87e03ccf-4842-45c3-b199-3817f8463103 --provider haproxy

DEBUG: keystoneclient.session REQ: curl -g -i -X POST http://10.1.1.36:9696/v2.0/lb/pools.json -H "User-Agent: python-neutronclient"
-H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token: {SHA1}8deb61cc7c8014a29d6ba13c01c81245de0c5fcb" 
-d '{"pool": {"lb_method": "ROUND_ROBIN", "protocol": "HTTP", "name": "foo", "admin_state_up": true, "subnet_id": "87e03ccf-4842-45c3-b199-3817f8463103", "provider": "haproxy"}}'

[coveralls]

Coverage increased (+0.002%) to 80.179% when pulling 300b597 on jtopjian:openstack-lbaas-pool-provider into c54bbac on rackspace:master.

[jrperritt]

Sorry for the delay. +2