requester: use a better error message when urls don't start with http/s

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
cloudmonkey: fix config change issue and credential usage issue

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
config: simply config reading and writing logic

- Treat all global variables, config dicts as readonly immutables
- Don't store anything outside method
- Remove need for first_time variable, checks for file before writing back

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>