Retry logic for 'st2client' and 'st2 pack'

[arm4b]

Improvement from the StackStorm/st2-packages#429

Most of the networking-related logic needs retrying mechanism. This is a rule of dumb in any package manager/client since in reality failures occur pretty frequently.

StackStorm shouldn't be an exception in following more reliable strategy.

TODO

1. Add retry, timeout, delay-on-retry logic to st2client httpclient
2. st2 pack retry logic for git cloning and any networking-related requests.

Resources

• https://github.com/StackStorm/st2/blob/master/contrib/packs/actions/pack_mgmt/download.py
• https://github.com/StackStorm/st2/blob/master/st2common/st2common/services/packs.py
• http://docs.python-requests.org/en/master/user/advanced/#timeouts
• http://docs.python-requests.org/en/master/api/#requests.adapters.HTTPAdapter
• DEFAULT_RETRIES = 0