update tag query param behavior to allow for additive tag specification

[edaniszewski]

@hoanhan101 brought up this point on usability:

specifying multiple tags, e.g. ?tags=system/type:fan,system/type:led will filter for devices which match all specified tags by design, so the above will not match anything because there is no device with two device types.

There likely will be cases where we want to be able to specify multiple tag groups in one request rather than having to split them across separate ones.

Right now, we can specify the tags query param multiple times, and those values just get included into a single filter, e.g. ?tags=system/type:fan&tags=system/type:led is the same as above.

For this issue, we should change the behavior where a single tags query param specified a single tag group to filter on. If multiple tags params are specified, the results of each group filter is joined and returned, so ?tags=system/type:fan&tags=system/type:led would return all fan-type devices and led-type devices.

[edaniszewski]

@hoanhan101 -- just providing an update since I know you have some work items (vapor-ware/synse-client-go#69) that are blocked by this

i've been slowly working through this one. it requires a bunch of updates to the tests, so its just taking me a while to go through and update everything and add in additional tests. I think I should have this completed by tomorrow.