Add mongo auth

[Lomonosow]

#13 Is work for me, but use mongo admin user. I`d tried to find another way but unsuccessfully)

[warrenvw]

@Lomonosow Thank you for your contribution. I'll be reviewing it shortly.

[Lomonosow]

I found some mistake in configuration, mongo want key at least 6 symbols length. I have fix it in my commit.

[arm4b]

@warrenvw Can you please drive this review, think about corner cases and check the PR?

From my point of view it would be nice to add a condition and set mongo username/password in st2.conf only if mongodb-ha.auth.enabled = true

[warrenvw]

Great work @Lomonosow. I manually tested this PR against our EKS cluster, and can only see a few things I'd do differently.

[warrenvw]

I'd appreciate if you could update the Changelog and bump the version in Charts.yaml. The Changelog should probably indicate that auth is now enabled by default.

Please note that I have not been able to successfully transition mongodb from auth disabled to auth enabled. Have you tried this? I'm attempting this particular scenario because auth is likely disabled on most installations... and now by default it will be enabled.

For example, in the st2actionrunner pod, I see:

OperationFailure: Authentication failed.

[Lomonosow]

For example, in the st2actionrunner pod, I see:

OperationFailure: Authentication failed.

I have reproduce it when I`d change mongodb admin password and tried to upgrade helm release. Mongodb chart does not recreate user if directory for db already exists (in mongodb persistent storage enabled by default and if you delete release by helm, persistent storage will not be deleted).