PowerVS create infra expects transit gateway location even when we pass existing transit gateway

[dharaneeshvrd]

When user tries to use existing transit gateway, location is not required as transit gateway name is unique across different locations.

[dharaneeshvrd]

/assign @aman4433

[openshift-ci]

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/assign @aman4433

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[dharaneeshvrd]

/cc @aman4433

[aman4433]

/assign