variable contender targets

[zeroXbrock]

• spam 'op-geth' when running opstack, instead of 'el'
• add --contender.target to override the spam target if desired

# spam op-geth
go run main.go cook opstack --contender

# spam "el" node instead of op-geth
go run main.go cook opstack --contender --contender.target el

also added a helper function svcManager.RunContenderIfEnabled() which DRYs out all the recipes running contender.