fix(put-policy): update policy without delete-create

[vpsx]

Previously Arborist's PUT policy endpoint would just delete and (re)create a policy. But this changed the policy's pkey in the db, which breaks links in [usr, grp, client]_policy. New PUT policy handler will just delete the relevant entries in the policy_[resource, role] tables and make new ones.

Additionally this PR:

1. Overwrites policy name in json with policy name from URL after unmarshaling is done, to correctly prefer the name from the URL

Expected behavior post-Arborist 3.0.0:

• PUT /policy endpoint deprecated and only PUT /policy/{policyID} used
• get policy name from url and ignore id field from request json if given

Behavior in the meantime (2.x.x):

• Both PUT /policy and PUT /policy/{policyID} supported
• If policy name in url, then overwrite policy name in request json, if latter is present; if policy name not in url, then use policy name from request json.

Bug Fixes

• fix PUT policy behavior so that it does not unexpectedly remove usr/grp/client-policy relationships
• lowercase id field name in unmarshalJSON's optional fields
• validate id field in policy validate fn (error if policy id not in policy json or is empty string)
• overwrite policy name from json if policy name was present in url (overwrite should be done after unmarshal, not before)

[coveralls]

Coverage decreased (-0.2%) to 74.784% when pulling 7cdf6fb on fix/policy into c58e30b on master.