update changelog for 0.15.2

CHANGELOG.md

+# v0.15.2 (2020-07-23)
+
+This minor release includes two new features:
+
+* Spack install verbosity is decreased, and more debug levels are added (#17546)
+* The $spack/share/spack/keys directory contains public keys that may be optionally trusted for public binary mirrors (#17684)
+
+This release also includes several important fixes:
+
+* MPICC and related variables are now cleand in the build environment (#17450)
+* LLVM flang only builds CUDA offload components when +cuda (#17466)
+* CI pipelines no longer upload user environments that can contain secrets to the internet (#17545)
+* CI pipelines add bootstrapped compilers to the compiler config (#17536)
+* `spack buildcache list` does not exit on first failure and lists later mirrors (#17565)
+* Apple's "gcc" executable that is an apple-clang compiler does not generate a gcc compiler config (#17589)
+* Mixed compiler toolchains are merged more naturally across different compiler suffixes (#17590)
+* Cray Shasta platforms detect the OS properly (#17467)
+* Additional more minor fixes.
+
 # v0.15.1 (2020-07-10)
 
 This minor release includes several important fixes: