MDSAuthCaps: use string and vector directly

MDSAuthCaps: use string and vector directly

Since std::string and std::vector are brought into the current namespace
at the beginning of MDSAuthCaps.cc, write "string" and "vector" instead
of "std::string" and "std::vector" respectively.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit 34f2243b8c89ed13760eb55994bef5f1a679ab24)

Conflicts:
	src/mds/MDSAuthCaps.cc: some code using string/vector was
	deleted by a different Reef backport due to which some patches
	from this commit weren't applicable.