Add option for retry number of connection attempts

This patch adds a new option (-r | --retries) to specify how many times the
client should attempt to connect to the Neutron server when using idempotent
methods (GET, PUT and DELETE). The patch also provides more user-friendly
message when it's impossible to connect to Neutron server from CLI and
ensures that connection-related exceptions are raised to the caller by
default when neutronclient is used as a library.

DocImpact
Closes-Bug: #1312225

Change-Id: Id74d7cf9a0e8c5d2cd3ee4851c883d5286bea19d
Merge "Add option for retry number of connection attempts"