[cuebot] DB optional env vars to streamline custom deployment (#2076)

**Summary**
Add the possibility to set optional env vars to connect to the db.

**Changes**
In the `opencue.properties` file, add 3 new environment variables to
simplify the deployment:

`CUEBOT_DB_URL`: URL of the Postgres database.
`CUEBOT_DB_USER`: the username to use to connect to the Postgres
database
`CUEBOT_DB_PASSWORD`: the password to use to connect to the Postgres
database

Those environment variables are optional. Default behavior stays
unchanged.

**Why**
Streamline custom config of cuebot and db setup on client side.

**Feedback**
Any documentation you would recommend updating?

Author: anton-ubi

cuebot/src/main/resources/opencue.properties

@@ -2,9 +2,9 @@ cue.proxy = tcp -h cuetest01-vm -p 9019 -t 10000:tcp -h cuetest02-vm -p 9019 -t
 spring.velocity.checkTemplateLocation=false
 
 datasource.cue-data-source.driver-class-name=org.postgresql.Driver
-datasource.cue-data-source.jdbc-url=jdbc:postgresql://dbhost/dbname
-datasource.cue-data-source.username=cue
-datasource.cue-data-source.password=password
+datasource.cue-data-source.jdbc-url=${CUEBOT_DB_URL:"jdbc:postgresql://dbhost/dbname"}
+datasource.cue-data-source.username=${CUEBOT_DB_USER:cue}
+datasource.cue-data-source.password=${CUEBOT_DB_PASSWORD:password}
 
 # HikariCP Configuration
 # Maximum lifetime of a connection in milliseconds (5 hours = 18000000 ms)