Allow loading a CA bundle from `trust-manager`

[reknih]

Is your feature request related to a problem? Please describe.
As described in #10, I want to use the chart with an external database. When connecting to the database, the server emits the following warning:

13:09:21.358 [warning] Description: 'Server authenticity is not verified since certificate path validation is not enabled'
     Reason: 'The option {verify, verify_peer} and one of the options \'cacertfile\' or \'cacerts\' are required to enable this.'

I want to securely connect to the server and validate its certificate.

Describe the solution you'd like
trust-manager is one of the preferred solutions to manage certificates in a Kubernetes cluster. Ideally, I could just specify the name of my Bundle in the values file to automatically have the necessary environment variables and volumes configured.

Describe alternatives you've considered
Allow the user to mount a CA volume via a YAML volume spec from the values file and configure the environment variables of the container accordingly or allow the user to load arbitrary volumes and document what extra environment variables they need to set such that the CA certificates are discovered.