bosh_login: Use val_from_yaml to get BOSh password
bosh_login: Use val_from_yaml to get BOSh password

So that we parse the YAML instead of trying to scrape strings from it. This
will be more reliable and help with the quoting problems described in
cdcab37.

jimconner/paas-cf,jimconner/paas-cf,jimconner/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,jimconner/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,jimconner/paas-cf,jimconner/paas-cf,alphagov/paas-cf,jimconner/paas-cf