Added ability to mount external PVCs to cdap deployments and statefulsets

[dvitiiuk]

There is a requirement to mount read-write storage for saving securestore file and custom jar for authorization. We are not able to use secretVolume and configMap because they are read-only and limited by maximal size of content.

So was decided to add property for mounting existing k8s PVCs to CDAP statefulsets and deployments.This feature has a similar design to secretVolume and configMap mounting features.

Example of usage:

spec:
  ...
  config:
    security.authorization.extension.jar.path: "/etc/securestorage/cdap-authorization-ldap-role.jar"
   ...
  pvcVolumes:
    task-pv-claim: "/etc/securestorage"

In this case pvc with name task-pv-claim will be mounted to /etc/securestorage for all CDAP pods.

Testing:

cdap-operator was built:
sudo docker build . -t cdap-operator-pvc:0.0.1
and used to setup file-based securestore and custom ldap based authorization on CDAP v6.5.0.

Unit testing:

go test ./... -coverprofile cover.out
?   	cdap.io/cdap-operator	[no test files]
?   	cdap.io/cdap-operator/api/v1alpha1	[no test files]
ok  	cdap.io/cdap-operator/controllers	8.509s	coverage: 61.5% of statements
?   	cdap.io/cdap-operator/controllers/cdapmaster	[no test files]

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