Don't use root_helper when it's not needed

LinuxBridge agent doesn't need to use root_helper when checking if
a device exists. Root priviledges are required to perform operations
inside a namespace and it's not the case here.

Change-Id: I12e0cde2f4dd3b3f8e7f5ed9e90e2805a4466aaa
Closes-bug: #1305800
Merge "Don't use root_helper when it's not needed"