Fix ycmd--locate-default-tags-file for case insensitive file-systems.
Fix ycmd--locate-default-tags-file for case insensitive file-systems.

On case insensitive file systems if we look for both small letters and
capitalized version of the same tags file name, we get incorrect result
because 'file-exists-p' is in that case also case insensitive. Introduce
a defcustom for default tags name.

abingham/emacs-ycmd,phst/emacs-ycmd,ptrv/emacs-ycmd,emacsmirror/ycmd,phst/emacs-ycmd,emacsmirror/ycmd,phst/emacs-ycmd,phst/emacs-ycmd,ptrv/emacs-ycmd,emacsmirror/ycmd,abingham/emacs-ycmd,emacsmirror/ycmd,abingham/emacs-ycmd,phst/emacs-ycmd,phst/emacs-ycmd,abingham/emacs-ycmd,emacsmirror/ycmd,ptrv/emacs-ycmd,emacsmirror/ycmd,abingham/emacs-ycmd,ptrv/emacs-ycmd,ptrv/emacs-ycmd,abingham/emacs-ycmd,ptrv/emacs-ycmd