Get sidekiq_pid value by reading sidekiq_config

[metavida]

Currently, even if I specify a pidfile value in my sidekiq.yml file, I'm forced to also set a sidekiq_pid value for capistrano-sidekiq. If I don't set a sidekiq_pid value, capistrano-sidekiq actively tells sidekiq to use capistrano-sidekiq's default pidfile location, ignoring my sidekiq.yml value.

This pull has capistrano-sidekiq pick a pidfile in the following order

1. A user-set sidekiq_pid value (plus any index number)
2. If a user sets a sidekiq_config value, any pidfile entry for the current sidekiq_env
3. If a user sets a sidekiq_config value, any global pidfile entry
4. The default pidfile location (plus any index number)

This Pull request should also address

• Issue pidfile being appended with -0 #134 - because the value from their sidekiq.yml will be honored
• @edslocomb's problems caused by Pull Change pidfile handling, always add index to pidfile name #116 [1][2]
• Partially addresses Issue pidfile handling #44 - So long a the user specifies unchanging pidfiles in the config file for each process, they'll be able to add processes as needed without worry about pidfile values changing. I'll be creating a new pull request in a bit that allows users to specify a config file per process.