Add REDIS_PASSWORD to server container and use it for readiness probe

[teamon]

Fixes #218

Changes proposed on the PR:

• Add REDIS_PASSWORD environment variable (via secret) to redis server container if auth: ... option is used
• Pass that variable to readiness probe
• It works both when the variable is set and when it's not

[chusAlvarez]

niiiiiice!
Thanks for the fixing :) :) :)

[zedtux]

@chusAlvarez If I'm not mistaking, you now have to build a new version of the operator Docker image, right?

Can someone please do it?

[ese]

@zedtux you have it available here docker pull quay.io/spotahome/redis-operator:v1.0.0-rc.5