Add ability to turn off some of the local repository mirrors

[peterfpeterson]

Rather than relying on network and such to fail quickly from servers that cannot be seen, add options to only include selected local mirrors. Since this happens at the cmake configuration stage, only the desired mirrors will be added to the list to search. The new mechanism defaults to using all mirrors.

Nothing was changed in buildscripts because it requires a mechanism to determine which build server is being used.

There is no associated issue.

To test:

Try the various options all, none, ornl, and ral and look for the cmake message about which mirrors are used. One can also search the CMakeCache.txt for the value of ExternalData_URL.

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