Reload configuration using the web api

[Abuelodelanada]

While executing _reload_config now we get:

could not reload configuration: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response'))

[dstathis]

We should make sure to fix this in both the k8s and machine charm.

[sed-i]

Need to make sure we're not opening a DoS door.
Perhaps upstream added the SIGHUP method by now.
...Unless gagent doesn't have a "enable admin api flag" to begin with.