Allow to create new PDB from an existing pdb file instead of pdbseed

[KTannenberg]

Allow to create PDB from a previously exported/unplugged <PDB_NAME>.pdb file instead of creating an empty PDB from pdbseed.

Our use-case:

1. Preparing usable database image requires importing data from dump and/or executing migration scripts (Flyway, Liquibase, etc; in-house solution in our case). In order to save on size of a final image it's more efficient to:
   1. Export pluggable database as a single <PDB_NAME>.pdb file
   2. Create a custom image that COPY this file
   3. Have user-script(s) that creates a new pluggable database from this file during first container startup

It would be nice to have ability to just upload custom <PDB_NAME>.pdb file into image and instruct container to create PDB with matching name upon startup by setting ORACLE_DATABASE property.

Currently an empty PDB will be created using pdbseed which then needs to be dropped and re-imported from <PDB_NAME>.pdb file in a user-script(s). This leads to a short time period in which empty PDB created from pdbseed can be observed by outside world as available to outside world until user-script(s) replace it with a correct one, in particular this affects container's healthcheck.