fixing typo to properly expand variable with multiple values

[lutierigb]

in case there are two default routes the quotation marks will concatenate the interface names. without the quotes, each interface will be treated as a separate value in the for loop. see http://b/336186274#comment25

[google-oss-prow]

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[jingyuanliang]

Can we add a comment saying $default_nic may be multi-line specifying multiple interfaces? In case we forget this and add the quotes back later.

[jingyuanliang]

#308 for a better version.

[lutierigb]

closing in favor of #308