master: fix "graph_period hour"

The "graph_period hour" setting does not work as expected, it stays "second".

Closes: #1305
Merge branch 'fix/1305-graph-period-hour-not-working' into stable-2.0
plugins: fix uninitialized $MUNIN::VERSION

Commit 733f01b1ad48c1e59f0804318a90615429f68e77 introduced a
specific user agent for munin plugins.

But it forgot to import the Munin::Plugins package.

Closes: D:717265
Merge branch 'fix/d/717265-use-uninit-val-const' into stable-2.0

Conflicts:
	plugins/node.d/nginx_request.in
	plugins/node.d/nginx_status.in