defaults should exist outside of config file

[cjestel]

Currently dbdeployer depends on default values existing for certain things. It's possible that people may attempt to strip down the config file to only what they want to override. If this were to happen we shouldn't break. I'm thinking it'd be good to have a defaults file that gets sourced prior to the config file so that we always have our defaults regardless of what is in the config file and it is just overridden if necessary.

[mattmc3]

Or, alternatively, use the bash default assignment syntax to assign a default on first use:

VAR1="${VAR1:-default value}". # if var1 exists, no-op; else assign default