provide configurable url for root for pgadmin

[jmccormick2001]

**What is the motivation or use case for the change? **

Configuring Kubernetes ingress to map the pgadmin container's root to a path under the cluster's root ("/pgadmin/") appears to result in http redirect errors from the python web application.

Describe the solution you'd like

Please provide parameter(s) for mapping the pgadmin website URL to a path other than the root ("/"). Also please consider defaulting to a location other than the root to avoid conflicts with other packages or services running in shared Kubernetes clusters.

• Operating System:
• Where is this running ( Local, Cloud Provider)
• Storage being used (NFS, Hostpath, Gluster, etc):
• Container Image Tag:
• PostgreSQL Version:
• Platform (Docker, Kubernetes, OpenShift): Kubernetes
• Platform Version:

Additional context
Add any other context or screenshots about the enhancement request here.

[jmccormick2001]

CrunchyData/postgres-operator#496

[jmccormick2001]

@kssikora here is the issue you created, we'll work it from the crunchy-containers project.

[cahoonpwork]

this will be in 2.3.1 that comes out late February.