Allows for dynamic urls for plugin routes. There are a few plugins
where the route url should be configurable and this change allows
using jsonData fields in the url field for a route in the
plugin.json file for a plugin.
In some setups (ex openshift), the Datasource will require Grafana
to pass oauth token as header when sending queries.
Also, this PR allow to send any header which is something
Grafana currently does not support.
After refactoring to be able to mock the client in
a test, the client wasn't a global variable anymore.
This change moves the client back to being a package-
level variable.
