provision/juju: support VPC on ELB creation

VPC requires subnet and security group parameters, and scheme to be
"internal".

To use VPC, three settings must be defined in the tsuru configuration
file:

	1. juju:elb-vpc-subnets (list). List of VPC subnet IDs to bind
	load balancers. There is no default value.
	2. juju:elb-vpc-secgroups (list). List of security group IDs to
	bind load load balancers. There is no default value.
	3. juju:elb-use-vpc (boolean). If true, will use the settings
	listed above when creating load balancers. The default value is
	false.

Related to #244.
queue: detect connection failures and reconnect

This may end in a infinite loop in future (if I can connect but cannot
execute list-tubes), but I'm not concerned right now.