Add environment variables to template rendering

[DaspawnW]

The credentials endpoint in Vault don't return for example the name of the database or the host information about the database. To have one file with all db related connection information it would be good to also include host information or database name information. Therefore the following change allows to render environment variables provided to the Pod, e.g.:

env:
  - name: HOST
    value: localhost:5432
  - name: DBNAME
    value: postgresql

spring.datasource.url: jdbc:postgresql://{{ .HOST }}/{{ .DBNAME }}
spring.datasource.username: {{ .Username }}
spring.datasource.password: {{ .Password }}

Output:

spring.datasource.url: jdbc:postgresql://localhost:5432/postgresql
spring.datasource.username: User
spring.datasource.password: VERYSECUREPASSWORD

[Joseph-Irving]

Hey thanks for this PR! looks like a good useful change, I left one nitpick

[DaspawnW]

I'd be very happy if you'd take over the merge. Thank you for the note :-)