How can I set the branch method used by glpk solver through pyomo?
To use --first as branch method
import pyomo.environ as pe
opt_solver = pe.SolverFactory("glpk")
opt_solver.options["first"] = ""
glpsol.exe --help provides the following options: 
Options specific to the MIP solver:
--nomip consider all integer variables as continuous (allows solving MIP as pure LP)
--first branch on first integer variable
--last branch on last integer variable
--mostf branch on most fractional variable
--drtom branch using heuristic by Driebeck and Tomlin
(default)