Don't overwrite SVN auth cache

From MRELEASE-497. Patch attached to that issue

When release-plugin commit to SVN, it overwrites the auth cache with username/password defined in the pom.xml.
I want to use a separate username `maven' for maven-related commits, and personal username `lenik' for personal development commits, when I start to using maven release plugin, it always overwrite the auth cache with user `maven' , and I must do a "Tortoise/Settings/Saved Data/Clear auth cache" before commit my personal changes.

A recommend solution:
add an option "--no-auth-cache" to svn commit command line will resolve this problem. with option --no-auth-cache, the existing auth cache is left unchanged after commited.
if there is an auth-cache option in pom.xml, say <auth-cache>false</auth-cache> under <scm> element, it should be false be default.