Fix parsing --drain-priority-config

The --drain-priority-config flag was only parsed if isFlagPassed()
returned true for it. However, isFlagPassed() would actually silently
never work. The implementation relied on walking the flags parsed by
the standard Go "flag" pkg. This seems like it would work since CA
defines its flags using the standard "flag" pkg. However, the flags
are then actually parsed and processed by the "github.com/spf13/pflag"
pkg, so isFlagPassed() could never see them.

This commit replaces removes isFlagPassed() and replaces the calls
with a pkg-provided pflag.CommandLine.Changed(). Unit tests are added
to verify that the flag is correctly parsed after this change.