cmd/bosun fixed tags parameter for silencing button on dashboard

[KozzyKoder]

Hello! Fixed small bug.

steps to reproduce:

1. open bosun dashboard
2. find any open incident
3. click on the incident description, an accordeon panel will expand on a dashboard page showing more info about the incident
4. click on any silencing button directly on dashboard page or just copy a link to the silencing page

expected:

silencing page will understand tags of the alert and fill them in the form

actual result:

when a link to the silencing page is formed we can see undefined instead of tag values, e.g.: https://bosun-host/silence?duration=2h&alert=some_alert&tags=undefined
on the opened silencing page you can see a red highlighted note on top of the page saying 'opentsdb: bad tag: undefined'

Thanks!