Replace service with worskpace after power-iaas plugin update

[Amulyam24]

What this PR does / why we need it:
With the power-iaas plugin version update to 1.0.0, replace service-instance references with workspace. This PR fixes the below error.

Plug-in 'power-iaas 1.0.0' was successfully installed into /root/.bluemix/plugins/power-iaas. Use 'ibmcloud plugin show power-iaas' to show its details.
++ jq -r '.[].crn'
++ ibmcloud resource service-instance e7207381-53be-4a6a-b2c1-127580e97466 --output json
+ CRN=crn:v1:bluemix:public:power-iaas:syd04:a/7cfbd5381a434af7a09289e795840d4e:e7207381-53be-4a6a-b2c1-127580e97466::
+ ibmcloud pi service-target crn:v1:bluemix:public:power-iaas:syd04:a/7cfbd5381a434af7a09289e795840d4e:e7207381-53be-4a6a-b2c1-127580e97466::
Error: unknown command "service-target" for "pi"
Run 'pi --help' for usage.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Replace service with worskpace after power-iaas plugin update

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[mkumatag]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Amulyam24, mkumatag

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