Read dedicated login creds for each platform

Previously we read the hardcoded `CCTRL_EMAIL`
and `CCTRL_PASSWORD` env variables as login credentials.

This introduces dedicated envs for login into exoscale, dotcloudng
or cnh platforms.

exoscale -> EXO_EMAIL, EXO_PASSWORD
dotcloudng -> DC_EMAIL, DC_PASSWORD
cnh -> CNH_EMAIL, CNH_PASSWORD
Merge pull request #38 from cloudControl/logincreds

Read dedicated login creds for each platform