Add support to Vault token

[djosemartine]

Spring cloud config server supports Vault as a backend.

It would be nice to be allowed to set the vault token as an option in the configurations, i.e, spring.cloud.config.token

In that way, the vault token could be provided by setting that option:
spring.cloud.config.token: vault-token

It would useful if that value can be overridden by an environmental variable such as SPRING_CONFIG_AUTH_TOKEN