Randomize pass if value is empty. Override if secret is specified

[scottrigby]

fixes: #139

Notes to reviewers

This PR also deprecates adminPassword with a deprecation note in values.yaml and _helpers.tpl that it will be removed in a future version.

For now the PR changes that to:

admin:
  password: ""
  existingSecret: ""

Mainly because I felt adminPasswordExistingSecret would be a bit long. But can change that back if you prefer.

To-do:

• If deprecated value is accepted, Update documentation elsewhere. eg, website and https://github.com/flanksource/mission-control README

[moshloop]

I think we can skip passing in a plaintext password, if you want to specify a password then you must create the secret before and set create: false

Suggested change

	admin:
	# If you leave this empty, and do not override with admin.existingSecret, a
	# random password will be generated for you.
	password: ""
	# Specifying an existing secret name will override the admin.password.
	# The secret key must be "admin-password"
	# For example, to create from a file:
	# echo -n 'S!B\*d$zDsb=' > ./admin-password.txt
	# kubectl create secret generic admin-password-secret --from-file=admin-password.txt
	existingSecret: ""
	adminPassword:
	secretKeyRef:
	create: true # set to false if you want to pass in an existing secret
	name: mission-control-admin-password
	key: password