ctx: add dry-run
cmd-install: impl. dry-run
cmd-remove: impl. dry-run
ctx: more impl. for dry-run
cmd-update: impl. dry-run
Merge pull request #14 from lhcb-org/dev/dry-run

Dev/dry run
yum: docstring
ctx: use provides
all: refactor use-provider into Client.FindLatestProvider
all: consistently use Client.FindLatestProvider (instead of FindLatestMatchingName)
cmd-deps: use latest-provider