Handle ZOWE_EXPLORER_FRAME_ANCESTORS undefined command line param

[NakulManchanda]

Issue has been reported by customers, when upgrdaded from 1.11 to 1.12 by applying PTF update, and if data changes are not applied manually, then user received following message in logs:

failed to process config                                           
TypeError: config.csp.frame-ancestorsÝ0¨.split is not a function   

This message is written by all three explorers as we are unable to start them.
Reason being we introduced new property
ZOWE_EXPLORER_FRAME_ANCESTORS in 1.12
This property is required to be present in instance.env with some valid value,
when undefined it is treated as boolean, which breaks the build.
It breaks when we try to call split function of strings on boolean value during explorer-ui-server startup

[NakulManchanda]

Support omission of this line from instance.env
https://github.com/zowe/zowe-install-packaging/blob/94d7b3f5cef53433800bf6a71619835752530b0b/scripts/instance.template.env#L39

[NakulManchanda]

@jordanCain i will apply following changes and send PR for review
so solution is:

• handle undefined better in server
• but also adding ZOWE_EXPLORER_FRAME_ANCESTORS to start.sh of explorers if undefined/missing in instance.env