Remove default-install of collectd

[avikivity]

Since bc7503f scyllatop no longer needs collectd. This allows us to drop collectd, which consumes a lot of CPU on large machines.

We can move collectd support (including the dependency on collectd) to a sub-package scylla-collectd that is not installed by default, and change the default configuration to disable collectd.

[avikivity]

/cc @syuu1228

[tzach]

@avikivity this is a breaking change for anyone who does use collectd.
We should add a 2.2 disclaimer that collectd is deprecated, and will be disable by default in the future.
In 2.3 (or 2.4) we can implement the change with a proper note.

[avikivity]

It is not a breaking change, we retain collectd support. It's just not default.

When upgrading, they'll need an additional step of yum install scylla-collectd and everything will continue working.

[avikivity]

2.2 and 2.3 will still ship with collectd enabled by default/

[glommer]

Why can't we ship collectd the same way we do today, but default-disable it in systemd ?

[avikivity]

It's nicer to pull in fewer dependencies.

In the past, EPEL's collectd pulled in qt and half a million xorg dependencies; this has since been correctd. Still, it's nicer not to pull in something you don't need.

btw, since we no longer need a local collectd, perhaps we don't need anything. Someone who wants collectd can just configure it to point to their remote instance.

It will mean an extra step for collectd users, but it will be worthwhile for them.