chart: add the ability to disable the memcached deployment

[antonosmond]

In some cases it can be useful to deploy flux without memcached e.g. when not using the image scanning features or if you want to use an external memcached service deployed separately from the flux chart.
This change adds the ability to disable the memcached deployment in the helm chart and / or override the memcached hostname.

[stefanprodan]

@antonosmond have you tested Flux without memcached? If I remember right, it will not sync with git without it.

[stefanprodan]

@antonosmond have you tested Flux without memcached? If I remember right, it will not sync with git without it.

[antonosmond]

@stefanprodan The change is more about using an external memcached.
In our use case, we have various "domains" each with multiple applications.
Each of those domains runs in its own namespace and we deploy a flux per domain/namespace.
This means we have like 10 flux deployments and we don't want a memcached deployment for each flux. A single instance of memcached should be sufficient.

[stefanprodan]

LGTM

Please squash the commits and rebase from master.
Thanks!