Add env_var and file as supported types for CI variables. Variables of
type file expose to users existing gitlab-runner behaviour - save
variable value into a temp file and set the path to this file in an ENV
var named after the variable key.
Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.
Fixes that make this work:
* A change in Ruby (ce635262f5)
requires passing in the exact required length for OpenSSL keys and IVs.
* Ensure the secrets.yml is generated before any prepended modules are
loaded. This is done by renaming the `secret_token.rb` initializer to
`01_secret_token.rb`, which is a bit ugly but involves the least impact on
other files.
