res_phoneprov: Create accessor for ast_phoneprov_std_variable_lookup

Based on feedback from Richard, I created an accessor for
res_phoneprov/ast_phoneprov_std_variable_lookup and added
load priority to AST_MODULE_INFO.

Tested-by: George Joseph
Tested-by: Richard Mudgett

Review: https://reviewboard.asterisk.org/r/4076/
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Merged revisions 425480 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 425481 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@425482 65c4cc65-6c06-0410-ace0-fbb531ad65f3