using user-agent in RBAC rules can lead to 500 errors

[arekinath]

Currently if I create an RBAC rule that's conditional on the "user-agent" header, any request that tries to run that rule which doesn't have a user-agent header sent causes a 500, which looks like this in the muskie logs:

    InternalError: an unexpected error occurred; caused by MissingConditionError: missing condition in context: "user-agent"
        at Server.authorize (/opt/smartdc/muskie/lib/auth.js:1172:26)
        at next (/opt/smartdc/muskie/node_modules/restify/lib/server.js:731:30)
        at f (/opt/smartdc/muskie/node_modules/once/once.js:16:25)
        at Server.storageContext (/opt/smartdc/muskie/lib/auth.js:1111:5)
        at next (/opt/smartdc/muskie/node_modules/restify/lib/server.js:731:30)
        at f (/opt/smartdc/muskie/node_modules/once/once.js:16:25)
        at /opt/smartdc/muskie/lib/common.js:547:17
        at /opt/smartdc/muskie/node_modules/vasync/lib/vasync.js:96:5
        at /opt/smartdc/muskie/lib/common.js:466:25
        at /opt/smartdc/muskie/lib/common.js:588:25
    Caused by: MissingConditionError: missing condition in context: "user-agent"
        at new MissingConditionError (/opt/smartdc/muskie/node_modules/mahi/node_modules/aperture/lib/errors.js:58:19)
        at Evaluator.evaluateCondition (/opt/smartdc/muskie/node_modules/mahi/node_modules/aperture/lib/evaluator.js:141:15)
        at Evaluator.evaluateOne (/opt/smartdc/muskie/node_modules/mahi/node_modules/aperture/lib/evaluator.js:67:14)
        at /opt/smartdc/muskie/node_modules/mahi/node_modules/aperture/lib/evaluator.js:82:22
        at Array.some (native)
        at Evaluator.evaluate (/opt/smartdc/muskie/node_modules/mahi/node_modules/aperture/lib/evaluator.js:81:22)
        at MahiClient.authorize (/opt/smartdc/muskie/node_modules/mahi/lib/client.js:596:24)
        at Object.authorize (/opt/smartdc/muskie/node_modules/libmanta/lib/auth.js:148:14)
        at Server.authorize (/opt/smartdc/muskie/lib/auth.js:1131:18)
        at next (/opt/smartdc/muskie/node_modules/restify/lib/server.js:731:30)

I think we should initialise the "user-agent" field to an empty string in the conditions if not present so that such rules don't cause 500 errors.