Add task retries in case of connection failure

[arm4b]

Closes #151 (Retry logic for the connection-oriented commands)

The most-frequent random error when installing StackStorm is related to downloading packages or similar download-oriented operations (connection closed, 50X, SSL broken pipe, GnuTLS recv error, etc). All temporary.

Adding retry on failure logic where applicable.

• [x] epel
• [x] mongodb
• [x] rabbitmq
• [x] st2repo
• [x] st2
• [x] nginx
• [x] st2web
• [x] nodejs
• [x] st2chatops
• [x] postgresql
• [x] st2mistral
• [x] bwc