DRILL-8232: Add support for user credentials to VaultCredentialsProvider (#2558)
* Add support for user credentials to VaultCredentialsProvider.
* Change LOGGER var to lower case.
* Convert VaultCredentialsProvider to AppRole authentication.
BREAKING. VaultCredentialsProvider was previously configured with
a static Vault token in a BOOT option. Now it is configured with a
static AppRole and secret in BOOT options and those are used to obtain
Vault tokens.
* Fox logic and style.
* Satisfy RAT.
