Allow existing PVC and PV to make it work with Rancher 2.0

[soriyath]

Hello All,

I've made some changes so that I can install the helm chart for mailu on my Rancher v2.3.4 environment.

In that regard, I have:

• added a few new parameters to the persistence parameters;
• kept the hostingPath persistence type per default;
• added the mailu.claimName convenience variable as an indirection to the PVC (defaulting to mailu-storage as per before);
• updated the README.md file with some more context.

Please note, in order to install it on my server, I've created a helm repo on my Github page.
You will see the artifact and the index.yaml, as well as some config files for GitHub page on the PR.
I can either remove them or we can delete them after the merge. I'm keeping it like this as there probably will be review comments for modifications.

Best Regards,

Soriyath

[micw]

Please remove the binary files from the built charts from the PR

We have 3 situations to handle:
hostpath: create PV and PVC that uses this PV
managed PVs: create PVC but no PV
(new) existing claim: use existing claim name, create neither PV nor PVC

Please edit pvc.yaml accordingly, I added comments there too

[micw]

Also please rebase against master, I have fixed travis builds for PRs

[soriyath]

@micw I've rebased against master and have opened this PR: #17
I would like my master branch to reflect the upstream branch.
Can we close this one and continue the discussion on PR 17?

Also, when linting, I get this error:
[ERROR] templates/: template: mailu/templates/mysql.yaml:1:57: executing "mailu/templates/mysql.yaml" at <.Values.database.mysql.host>: nil pointer evaluating interface {}.host

I think it's unrelated to my changes, is it?