policy provider prevents updating an existing policy

[brendenyule]

The set action in policy provider prevents a policy from being set if it already exists.
I think expected behavior would be to let the policy be updated if it already exists.

rabbitmqctl set_policy is already an idempotent command and handles this out of the box.

Casual proof the rabbitmq set_policy is idempotent:

user@ip:~$ sudo rabbitmqctl list_policies
Listing policies ...
/	mps-ha-all	all	^	{"ha-mode":"all"}	0

user@ip:~$ sudo rabbitmqctl set_policy mps-ha-all "^" '{"ha-mode":"all"}' --priority 0
Setting policy "mps-ha-all" for pattern "^" to "{\"ha-mode\":\"all\"}" with priority "0" ...

user@ip:~$ sudo rabbitmqctl list_policies
Listing policies ...
/	mps-ha-all	all	^	{"ha-mode":"all"}	0

[michaelklishin]

Should be fixed in #425, thank you.