Fix typos in code comments & docstrings

Conflicts:
	ceilometer/storage/mongo/utils.py

Change-Id: Ia5937026018bb5441b7a5b6d82ce074889dfe78b
Use hmac.compare_digest to compare signature

hmac.compare_digest must be used to compare sample
signature to avoid timing-attack.

We also provide a best-effort version of hmac.compare_digest
for python version that doesn't have the C version.

Closes bug #1332390

Change-Id: Ia313c98b7cccf0e8aed4fb933292bd7e6141b801
Merge "Fix typos in code comments & docstrings"