hardcode secret name gitops-token in build-push-promote-pl pipeline

[tam512]

Currently users need to use GHE secret name gitops-token to run pipeline build-push-promote-pl. Please fix this hardcoded name.

If users are not using the secret name gitops-token, it git-source task would fail with the following error:

{"level":"error","ts":1593038246.5568674,"caller":"git/git.go:41","msg":"Error running git [fetch --recurse-submodules=yes --depth=1 origin eeccc0a3a38edabb4547bc53ed2c949897ef7874]: exit status 128\nfatal: could not read Username for 'https://github.ibm.com': No such device or address\n","stacktrace":"github.com/tektoncd/pipeline/pkg/git.run\n\t/go/src/github.com/tektoncd/pipeline/pkg/git/git.go:41\ngithub.com/tektoncd/pipeline/pkg/git.Fetch\n\t/go/src/github.com/tektoncd/pipeline/pkg/git/git.go:93\nmain.main\n\t/go/src/github.com/tektoncd/pipeline/cmd/git-init/main.go:53\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:203"}
{"level":"error","ts":1593038246.9485033,"caller":"git/git.go:41","msg":"Error running git [pull --recurse-submodules=yes origin]: exit status 1\nfatal: could not read Username for 'https://github.ibm.com': No such device or address\n","stacktrace":"github.com/tektoncd/pipeline/pkg/git.run\n\t/go/src/github.com/tektoncd/pipeline/pkg/git/git.go:41\ngithub.com/tektoncd/pipeline/pkg/git.Fetch\n\t/go/src/github.com/tektoncd/pipeline/pkg/git/git.go:96\nmain.main\n\t/go/src/github.com/tektoncd/pipeline/cmd/git-init/main.go:53\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:203"}
{"level":"warn","ts":1593038246.9486105,"caller":"git/git.go:97","msg":"Failed to pull origin : exit status 1"}
{"level":"error","ts":1593038246.9507523,"caller":"git/git.go:41","msg":"Error running git [checkout eeccc0a3a38edabb4547bc53ed2c949897ef7874]: exit status 128\nfatal: reference is not a tree: eeccc0a3a38edabb4547bc53ed2c949897ef7874\n","stacktrace":"github.com/tektoncd/pipeline/pkg/git.run\n\t/go/src/github.com/tektoncd/pipeline/pkg/git/git.go:41\ngithub.com/tektoncd/pipeline/pkg/git.Fetch\n\t/go/src/github.com/tektoncd/pipeline/pkg/git/git.go:99\nmain.main\n\t/go/src/github.com/tektoncd/pipeline/cmd/git-init/main.go:53\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:203"}
{"level":"fatal","ts":1593038246.9507914,"caller":"git-init/main.go:54","msg":"Error fetching git repository: exit status 128","stacktrace":"main.main\n\t/go/src/github.com/tektoncd/pipeline/cmd/git-init/main.go:54\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:203"}