Pass additional ssh key to the script. Extracted ssh values to global ci vars

[antgamdia]

Description of the change

This PR tries to solve a lack of permission issue since the default ssh key loaded is not valid for the forked repository.
At the same time, I've extracted the ssh-related params to CI variables, as passing manually the values is so error-prone

Benefits

Syncing charts working... hopefully

Possible drawbacks

N/A

Applicable issues

N/A

Additional information

An alternative is to define "labeled" host like:

Host github-as-kubeappsbot
  HostName github.com
  User git
  IdentityFile ~/.ssh/id_rsa.xxxxx
  IdentitiesOnly yes

But since we just need permission for pushing, it's clearer to just pass the key explicitly

[antgamdia]

Let me rubber-stamp this PR since it just implies changes on the CI

[absoludity]

+1