Switch from Personal Access Token to a Deploy Key

[captn3m0]

From the README:

# This should be a personal access token made by a member of organization on his/her
# behalf who can read the Keyhouse repository. Go to this
# https://github.com/settings/tokens/new?description=Keyhouse%20Token&scopes=repo
# to make a new token with correct scopes.
token = 'secret_token'

Using a PAT is riskier, since it gives complete user-rights on all repos for the user who creates the token. Putting such a token across multiple servers is not a great idea.

Instead, support a Identity File in the config, and use git clone over SSH with a GitHub Deploy Key instead.

[twaritwaikar]

cc @kanav99